Compounds of the formula (I), (Ia) or (Ib) wherein A1, and A− is for example (II) is 1 or 2; X is C1-C4 alkylene or CO; Y is for example O, O(CO), O(CO)O, R1 is for example hydrogen, d-dsalkyl, C3-C30cycloalkyl, phenyl, naphthyl, anthracyl, phenanthryl, biphenylyl, fluorenyl or C3-C20heteroaryl, all of which optionally are substituted; R2 and R3 for example are C1-C10haloalkylene which is optionally substituted, or R2 and R3 are phenylene, which optionally is substituted; R4 is a group (A) or a group (B); R5 and R6 for example are C1-C20alkyl; or R4 and R5 or R4 and R6 together form a straight-chain C2-C6alkylene, R5 and R6 together form a straight-chain C2-C6alkylene; R7, R8, R9 and R10 ifor example are C1-C20alkyl; M for example is C1-C20alkylene, C2-C20alkenylene, C2-C20alkynylene; R25 and R26 are for example hydrogen, C1-C20alkyl; R27, R28, R29, R30 and R31 are for example hydrogen, C1-C20alkyl, C2-C20alkenyl, C3-C20cycloalkyl, or two radicals R27 and R28, R28 and R29, R29 and R30 and/or R30 and R31 together form a straight-chain C2-C6alkylene, or R25 and R27 together form 1,2-phenylene, R33 and R34 for example are hydrogen, C1-C20alkyl; R35, R36 and R37 for example are hydrogen, C1-C20alkyl; are suitable as thermo-acid generators.