This study examines the fundamental and practical aspects of C-1-C-4 hydrocarbons separation (pure or in binary or ternary mixtures), using highly permeable glassy 1,2-disubstituted polyacetylenes. Poly(4-methyl-2-pentyne) (PMP) was chosen as the main object of study. An attempt is made here to carry out a systematic comparative study of the PMP permeability target parameters using correlation detection and a thermodynamic gas/polymer model. A non-isothermal application method was used, which allowed us to work with each gas individually and with various mixtures to obtain an experimental value for the dependence on temperature of the permeability of C-1, C-3, C-4 hydrocarbons. This is the first time that this method has been used for the study of glassy polymers with high free volume. An interesting phenomenon of an increase in non-additive separation selectivity alpha was shown (for a value of alpha of between similar to 10 and similar to 8000 units) with increase of butane activity (p/p(sat)) from 0.02 to 0.94 in the feed gas mixture. In the current study, an explanation is given for the increase in non-additive selectivity for PMP membrane separation. Modelling was carried out of the separation process of a binary butane/methane mixture using this non-additive phenomenon, which gives rise to concentrated components.