A new resist system is described which undergoes spontaneous relief image formation. The resist is formulated from end capped poly(phthaladehyde), PPA, and a cationic photoinitiator such as a diaryliodonium or triarylsulfonium metal halide. The extreme sensitivity of the resist is the result of designing for chemical amplification. The desired amplification results from the fact that photolysis of the sensitizer generates acid which catalyzes main chain cleavage of the polyaldehyde. The uncapped polymer is thermodynamically unstable with respect to reversion to monomer at room temperature so a single acid catalyzed scission results in complete depolymerization to volatile monomer. A single radiochemical event is thereby amplified in the sense that it produces an enormous number of subsequent chemical transformations. PPA/onium salt resist films are so sensitive that exposure to low doses of e‐beam, X‐ray or ultraviolet radiation results in complete self development without post‐exposure processing of any kind. The exposed area simply vaporizes. Copyright © 1983 Society of Plastics Engineers, Inc.