A positive-acting photoresist composition comprising: (i) an alkali-soluble binder comprising a plurality of phenol groups, (ii) an iodonium salt of the general formula: CHEM in which; Ar1 and Ar2 independently represent aromatic or heteroaromatic groups comprising of up to 10 ring atoms, which groups may optionally be linked together to include the iodine atom within a ring structure and, X(-) is an anion such that HX is an acid of pKa not greater than 3, and, (iii) an acid cleavable compound having a nucleus of general formula (I): CHEM in which; n is 0, 1, 2 or 3, each R1, R2 and R3 independently represents hydrogen or an alkyl group comprising up to 4 carbon atoms, R4 and R5 independently represent hydrogen or one or more ring substituents selected from Cl, Br, F, alkyl groups comprising up to 4 carbon atoms, alkoxy groups comprising up to 4 carbon atoms and combinations thereof, and A represents a bond or a divalent linking group containing up to 10 atoms other than hydrogen. The compositions of the invention are intended primarily for use in the preparation of lithographic printing plates, although they may also be used on positive photo-resists for the preparation of metal clad printed circuit boards and other integrated circuitry, proofing sheets etc.
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