Abstract: A hydraulic fluid composition for power steering is disclosed, which comprises (a) at least one phosphorus containing compound selected from the group of alkyl, or alkyl substituted or unsubstituted phenyl phosphorus acid compounds, alkyl, or alkyl substituted or unsubstituted phenyl phosphorus thioacid compounds, alkyl, or alkyl substituted or unsubstituted phenyl phosphorus dithioacid compounds, and (b) one or more thiadizaole derivatives represented by the formula: ##STR1## where R.sub.1 and R.sub.2 independently denote a linear or branched alkyl group having 1 to 12 carbons. The composition enables hydraulic systems to be used for longer times without accelerating deterioration of rubber materials employed in the hydraulic system.