本文在25℃下分别测定了正己酸和正庚酸在纯水和小离子盐水溶液中的溶解度.标出了弱酸未解离部分的活度系数f_u,作出氯化锂等八种盐的lgf_u～Cs图,均呈直线关系.计算并比较了乙酸等五种脂肪酸在小离子盐水溶液中的盐效应和盐析常数的各种理论值. In this paper, the solubility of n-caproic acid and n-heptanoic acid in pure water and small ion salt solution were measured respectively at 25 ℃ .The activity coefficient f_u of the undissociated part of weak acid was plotted and the lgf_u ~ Cs, all showed a linear relationship.The various theoretical values ​​of salt effect and salting-out constant of five kinds of fatty acids such as acetic acid in small ion salt solution were calculated and compared.