The influence of atmospheric humidity on the photochemically induced cationic polymerisation of epoxy groups in different chemical environments is shown. With increasing humidity, the polymerisation rate of glycidylethers decreases, whereas for epoxides with endocyclic epoxy groups the rate of polymerisation increases and the rate for epoxides with exocyclic epoxy groups is nearly not influenced. The reasons for the different behaviour are energetic differences for the reactions between the positively charged chain ends with water and the following proton transfer. Furthermore the reaction rate is influenced by the different stereochemistry of the attack of the next monomer.