Deep UV photoresists are designed to be used in the manufacturing of highly integrated chips (>16 Mbit). They differ from the conventional photoresists in their principal chemistry. The vast majority of positive deep UV resists are based on protected poly(hydroxystyrene) resins and photochemical acid generators (PAG). They rely on photochemically induced acid-catalyzed reactions (chemical amplification) to generate the desired pattern and meet the high-sensitivity requirements. It turned out that the type of the acid labile protective group is of paramount importance for the performance of the resists. It has to be stable enough not to be cleaved by the weakly acidic phenol at room temperature, but has to be labile enough to be cleaved readily even at the top of the resist where portions of the generated acid may be neutralized by airborne bases. Selection criteria for useful groups and the performance of the very well suited protective group methoxy(tetrahydropyran) are described in this paper.