Novel phosphonium salts of the general formula ##STR1## wherein R1 and R2 each is a hydrogen atom or a hydrocarbon group of 1 to 12 carbon atoms which may optionally be substituted; R3 is a hydrogen atom or a hydrocarbon group of 1 to 5 carbon atoms which may optionally be substituted; R4, R5 and R6 each is a hydrocarbon group of 1 to 8 carbon atoms which may optionally be substituted; X is a hydr