Provided are: a water-soluble composition which has excellent storage stability and is adaptable to a wide range of light sources and capable of forming a highly fine pattern; a method of producing a cured product of the same; a cured product of the same; and an acylphosphinate. The water-soluble composition contains: an acylphosphinate (A) represented by Formula (I) below, wherein X1 represents an aryl group having 6 to 15 carbon atoms; X2 represents a linear alkyl group having 1 to 8 carbon atoms or the like; Am+ represents an alkali metal ion or the like; and m represents a number of 1 to 3; and a compound (B) having a group represented by Formula (II) below, wherein R1 represents a hydrogen atom or the like; Z1 represents an oxygen atom or the like; R2 represents a hydrogen atom or the like; Z2 represents an alkylene group having 1 to 6 carbon atoms; n represents a number of 0 to 30; * means a bond; and, when the compound (B) has plural groups represented by Formula (II), plural R1′s, Z1′s, Z2′s and n's are each optionally the same or different.