A method for predicting the optimal salinity of individual members of a class of internal olefin sulfonate compositions in a mixture containing the individual member, a brine and hydrocarbons, wherein the internal olefin sulfonate compositions comprise a mixture containing hydroxy alkane sulfonates and alkene sulfonates, the method including: (a) determining the correlation between a weight ratio of hydroxy alkane sulfonates to alkene sulfonates of the individual members and separately the optimal salinity of individual members in a mixture containing the individual member, a brine and the hydrocarbons on the basis of two or more reference members of the class of internal olefin sulfonate compositions, each reference member having a different known weight ratio of hydroxy alkane sulfonates to alkene sulfonates; and (b) using the correlation to predict: b1) the optimal salinity of a further member of the class of internal olefin sulfonate compositions, which further member contains a weight ratio of hydroxy alkane sulfonates to alkene sulfonates different from the reference members; or b2) the required weight ratio of hydroxy alkane sulfonates to alkene sulfonates of an individual member at which weight ratio of hydroxy alkane sulfonates to alkene sulfonates the presence of the individual member in a mixture of the individual member, a brine and hydrocarbons, of which the salinity of the mixture is known, may result in a microemulsion with bicontinuous hydrocarbons and water phases, wherein the correlation is linear, exponential or logarithmic and is characterized by a function: OS = f x, wherein: OS = optimal salinity [wt% TDS or equivalent wt% NaCl]; and x = weight ratio of hydroxy alkane sulfonates to alkene sulfonates [-], wherein said hydroxy alkane sulfonates include hydroxy alkane sulfonates, hydroxy alkene sulfonates, hydroxy di-alkene sulfonates, but exclude any di-sulfonate species, and wherein said alkene sulfonates include mono-alkene sulfonates and di-alkene sulfonates, but exclude any di-sulfonate species and hydroxy alkane sulfonates as defined above. (a) determining the correlation between a weight ratio of hydroxy alkane sulfonates to alkene sulfonates of the individual members and separately the optimal salinity of individual members in a mixture containing the individual member, a brine and the hydrocarbons on the basis of two or more reference members of the class of internal olefin sulfonate compositions, each reference member having a different known weight ratio of hydroxy alkane sulfonates to alkene sulfonates; and (b) using the correlation to predict: b1) the optimal salinity of a further member of the class of internal olefin sulfonate compositions, which further member contains a weight ratio of hydroxy alkane sulfonates to alkene sulfonates different from the reference members; or b2) the required weight ratio of hydroxy alkane sulfonates to alkene sulfonates of an individual member at which weight ratio of hydroxy alkane sulfonates to alkene sulfonates the presence of the individual member in a mixture of the individual member, a brine and hydrocarbons, of which the salinity of the mixture is known, may result in a microemulsion with bicontinuous hydrocarbons and water phases, wherein the correlation is linear, exponential or logarithmic and is characterized by a function: OS = f x, wherein: OS = optimal salinity [wt% TDS or equivalent wt% NaCl]; and x = weight ratio of hydroxy alkane sulfonates to alkene sulfonates [-], wherein said hydroxy alkane sulfonates include hydroxy alkane sulfonates, hydroxy alkene sulfonates, hydroxy di-alkene sulfonates, but exclude any di-sulfonate species, and wherein said alkene sulfonates include mono-alkene sulfonates and di-alkene sulfonates, but exclude any di-sulfonate species and hydroxy alkane sulfonates as defined above. b1) the optimal salinity of a further member of the class of internal olefin sulfonate compositions, which further member contains a weight ratio of hydroxy alkane sulfonates to alkene sulfonates different from the reference members; or b2) the required weight ratio of hydroxy alkane sulfonates to alkene sulfonates of an individual member at which weight ratio of hydroxy alkane sulfonates to alkene sulfonates the presence of the individual member in a mixture of the individual member, a brine and hydrocarbons, of which the salinity of the mixture is known, may result in a microemulsion with bicontinuous hydrocarbons and water phases, OS = optimal salinity [wt% TDS or equivalent wt% NaCl]; and x = weight ratio of hydroxy alkane sulfonates to alkene sulfonates [-], wherein said hydroxy alkane sulfonates include hydroxy alkane sulfonates, hydroxy alkene sulfonates, hydroxy di-alkene sulfonates, but exclude any di-sulfonate species, and wherein said alkene sulfonates include mono-alkene sulfonates and di-alkene sulfonates, but exclude any di-sulfonate species and hydroxy alkane sulfonates as defined above.