Perovskite solar cells based on organolead halides such as CH₃NH₃PbX₃ (X = Cl, Br, and I) have rapidly established themselves as the frontrunners among emerging photovoltaic technologies. However, their commercial application has been hindered to date in part due to their susceptibility to degradation by UV radiation or heat in the presence of moisture. Herein we investigate the relationship between the physical properties of several polymer encapsulants (poly(methylmethacrylate) (PMMA), ethyl cellulose, polycarbonate and poly(4-methyl-1-pentene)) and their ability to function as barrier layers to improve the stability of CH₃NH₃PbI_3−xCl_x films under prolonged thermal degradation at 60 °C, 80 °C and 100 °C. In all cases, polymer-coated CH₃NH₃PbI_3−xCl_x films showed retarded thermal degradation compared to the uncoated films, as indicated by the quantitative decay of the perovskite band edge in the UV/Vis absorption spectrum and the appearance of PbI₂ peaks in the powder X-ray diffraction pattern. However, the extent of this reduction was highly dependent on the physical properties of the polymer encapsulant. Notably, PMMA-coated CH₃NH₃PbI_3−xCl_x films showed no visible signs of degradation to PbI₂ after extended heating at 60 °C. However, concomitant studies by epifluorescence microscopy (FM) revealed deterioration of the CH₃NH₃PbI_3−xCl_x film quality, even in the presence of a polymer-coating, at much shorter heating times (29 h), as evidenced by quenching of the film fluorescence, which was attributed to grain aggregation and the formation of associated non-radiative trap sites. Since grain aggregation occurs on a shorter timescale than chemical degradation to PbI₂, this may be the limiting factor in determining the resistance of organolead halide perovskite films to thermal degradation.