To obtain the subject new salt useful as a raw material for producing cationic polymers for coagulants, soil conditioners, electrodeposition coatings, etc., by reacting a triorganophosphine with an unsaturated carboxylic acid, treating the reaction product with an acid and subsequently reacting the reaction product with an acrylate compound.

A new unsaturated phosphonium salt of formula I [R1 to R3 are each a 1-8C alkyl; B is a group of formula II (Y is H, methyl); X is H, a 1-4C alkyl, phenyl; A is an organic acid anion, an inorganic acid anion]. the salt of formula I can have the counter anion of a desired organic or inorganic acid, and is useful as a raw material for producing cationic polymers, etc., used for coagulants, dyeing-improving agents, antistatic agents, soil conditioners, conductive processing agents, paper-reinforcing agents, cation electrodeposition coatings, etc. The compound of formula I is obtained by reacting a triorganophosphine of formula III with an unsaturated carboxylic acid of formula IV, reacting the obtained phosphonium salt having a betaine structure with an organic or inorganic acid of the formula: H+A- and subsequently reacting the reaction product with a compound of formula V (D is a group of formula VI, etc.).