as a clyde I can easily explain how to gain weight without gaining muscle mass in one word . . . cheetos. seriously though, I was trying to establish some baseline understanding of the OPs current body composition. for example, when I started working out seriously last year, I lost weight and gained muscle at the same time. holy grail, I know, and it was only because I was experiencing newbie gains, but I was able to lose weight and gain muscle at the same time. to a certain extent, if you are over say 20-25% body fat, this is possible for anyone if they follow a structured plan and choose their calorie composition wisely.