Provided are: a water-soluble composition which has excellent storage stability, is applicable for a wide variety of light sources, and can be used to form a highly fine pattern； a production method for a cured product of the water-soluble composition； a cured product of the water-soluble composition； and an acyl phosphinate. This water-soluble composition contains an acyl phosphinate (A) represented by general formula (I) (in the formula, X1 represents an aryl group having 6-15 carbon atoms, X2 represents a straight-chain alkyl group or the like having 1-8 carbon atoms, Am+ represents an alkali metal ion or the like, and m represents a number of 1-3), and a compound (B) having a group represented by general formula (II) (in the formula, 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 1-6 carbon atoms, n represents a number of 0-30, * indicates a bond, and, in the case where there are a plurality of the groups represented by general formula (II), the respective R1's, X1's , X2's , and n's existing in a quantity of more than one, may be the same or different from each other).本發明係提供保存穩定性為優異、能夠適應於廣範圍光源並可形成高精細圖型之水溶性組成物、其硬化物之製造方法、及其硬化物、以及醯基膦酸鹽。 其含有：下述一般式(I)所表示之醯基膦酸鹽(A)、與具有下述一般式(II)所表示之基之化合物(B)，(式中，X1為表示碳原子數6~15之芳基，X2為表示碳原子數1~8之直鏈烷基等，Am+為表示鹼金屬離子等，m為表示1~3之數)，(式中，R1為表示氫原子等，Z1為表示氧原子等，R2為表示氫原子等，Z2為表示碳原子數1~6之伸烷基，n為表示0~30之數，＊意味著鍵結部，若具有多個複數的一般式(II)所表示之基之情形時，複數存在的R1、X1、X2、n係可分別為相同或相異)。
