Liquid mixtures of trialkylphosphine oxides, useful as antistats in thermoplastic polymers, are obtained by alkylating excess phosphine with a mixture of higher .alpha.-olefins to give a mixture of higher alkylated phosphines and unreacted phosphine and alkylating that mixture with a lower .alpha.-olefin. This method provides a larger percentage of higher alkyl groups and a lower percentage of lower alkyl groups compared to alkylating the phosphine with all of the mixed .alpha.-olefins in a single reaction. The shift in higher alkyl groups decreases the volatility of the trialkylphosphine mixture and is an advantage when preparing plastic compositions at higher formulation temperatures.