The influence of the experimental conditions used to prepare films of cis/trans poly(tert-butylacetylene) [PTBA] on density and physical aging phenomena have been investigated. Films with different free volumes were obtained by casting from solvents with different thermodynamic characteristics. In order to choose the most appropriate solvents, the solubility parameter of PTBA was determined both through a calculation based on group contribution methods and by viscosity measurements in different solvents. Films from solvents with different solubility parameters have been prepared under controlled conditions. On increasing evaporation time and polymer-solvent affinity, higher density values and, consequently, higher chain packing and lower free volume are achieved. Aging phenomena, studied by dilatometric and calorimetric techniques, occur whenever the films undergo thermal treatment, indicating that densification takes place as a result of enhanced chain mobility at the elevated temperatures of the thermal treatment protocol.Most properties of the substituted polyacetylenes (PA), such as permeability to penetrants, solubility, and thermal stability are strongly related to the number and/or steric hindrance of the substituents (1-4). However, these properties cannot be related in a straightforward fashion to the kind of substituents or to the mono-or doublesubstitution along the main chain. In fact, the configuration of the repeat units and, consequently, the chain conformation and packing have to be taken into account and might be responsible for the properties of different polymers. For instance, it has been shown that structural differences originating from different cis/trans tactic ratio in