Pyrolysis of Bulgarian oil shale was studied by using a diamond anvil cell, which allows one to observe directly the conversion of organic matter to mobile oily liquid, solid residue and gas. This method gives the possibility to detect very precisely the temperatures of kerogen transformation during pyrolysis. It was observed that most of the gaseous products were generated later than oily liquids, most probably by cracking processes of liquid products. Due to the strong connection between mineral and organic matter, the generation of liquids and gases from oil shale occurred at higher temperature. Light intensity measurements were used to follow quantitatively the rate of reaction.