Thermal degradation of the low density polyethylene (LDPE), polypropylene (PP) and the municipal waste plastics was investigated. The thermo-catalytic degradation of LDPE and PP was studied in the presence of the following catalysts: four different types of montmorillonite: K5, K10, K20, K30 and - for comparison - zeolites (natural - clinoptilolite, YNa+ and YH+). Thermal analyses TG-DTA-MS of polymers and polymer-catalyst mixtures were carried out in an argon flow atmosphere in isothermal and dynamic conditions. The following order was found: in lowering the reaction temperature for LDPE degradation YH+ > mK5 > mK20 = mK30 >mK10 > NZ > YNa+; for PP degradation: mK20 > mK5 = mK30 >mK10 > YH+ > NZ > YNa+. The activity tests were carried out in a stainless steel batch reactor under atmospheric pressure in a wide temperature range of up to 410°C, and using the atmosphere of argon flow. The liquid products were analysed by the GC-MS method. The hydrocarbons in the liquid products from thermal degradation of polymers were broadly distributed in the carbon fractions of C8 to C26 - for LDPE and C6 to C31 for PP.