I ordered these to go with the catalyst pants for a base layer I’m glad I did they fit great.

I purchased these boot top longjohns to pair with my Corrugate Guide Pants, in the hopes of ditching the bulky pajamas I had rigged to help with deal with the cool spring mornings hunting Ontario Turkey. Morning temperatures on opening day dipped down below -6°C (20°F) and the airy Corrugate Guide Pant was not cutting it on its own, so I ordered these, and recieved them the next day. Amazing experience with First Lite right off the bat.
I came in from hunting my home property around noon to try them on and I have to admit, my initial impressions were skeptical. They felt lighter than I expected, and the fit was a bit looser than I'm used to. As I wore them that day, hiking back up the rocky ridges to where the turkeys were hanging out, I thought that there was no way these were going to keep me warm the next morning. They breathe way too well, and no matter how hard I pushed my legs were cool and dry. Determined to test them fully, I suited up the next morning and headed out before first light. Temperatures were a little warmer hovering around -2°C (28°F). Boy was I ever wrong.
This was one of my first times using a merino base layer pant and I didn't realize how well this blend could regulate body temperature. I was warmer with these than I was with any amount of bulky pajamas jammed down my pants legs. And the beauty of it was that it kept going with me all day long. The fit was lose enough to allow for another thin layer underneath without being too tight, but tight enough to feel like a second skin after getting used to the cool ring around my legs where my socks didn't quite make to top of the bottoms.
I can see myself wearing these as my Baselayer, coupled with added layers during colder months, from spring Turkey hunting, right through to late season. They breathe so well I might end up using them for day to day life when a light pant just isn't enough.

I know most people read these to figure out fit. I’m 5’11” 185 lbs with a bodybuilder frame. Pretty thick up top and through the butt and legs with a 32 waist. I went with a Medium in both the kiln quarter zip as well as the kiln boot top bottoms. again, perfect fit. I plan on using these as my first, next-to-skin base layer, so I wanted a more snug fit. I got exactly that but not overly tight. With my frame, if I was going to wear a different base later and use this as my 2nd base layer, I would get a large. Hope this helps someone.

These things are perfect for those cold mornings on the mountain! Super warm, but still breathable! I always hated the added bulk of standard thermals running down into the boot, so the boot-length design of these is ingenious. Between the sock and the bottom of the kilns, everything stays insulated and warm. At the same time, a brisk hike to a glassing point doesn't get you all sweaty and overheated just so you freeze again later...these things still wick moisture away like merino does so well. I was hunting with 2 buddies on a frigid morning with serious wind...I was the only one wearing these, and I was the only one not freezing!

These definitely run large. I’m 6’2”, 225 lbs, wear 36 x 34 pants. I initially ordered XL based on the sizing chart. After I tried them on I called to make sure I wasn’t crazy. Returned for L, which fit nicely. They still aren’t super snug, so if you’re used to synthetic compression type base layers they will feel loose. The length is nice, your socks and the base layer aren’t competing for space. Haven’t gotten out in really cold weather yet, but so far so good.