The apparent molal volumes ϕ_V of a series of homologous tetraalkylammonium chlorides, bromides and iodides have been measured precisely by means of a differential buoyancy technique, down to 0.01 m. The values of ϕ_V extrapolated to infinite dilution give partial molal volumes V^°_s which are almost additive for successive homologues and enable the absolute individual partial molal volumes of the halide ions to be estimated by a new principle to an accuracy of about 0.5 ml mole^–1. The concentration dependence of ϕ_V and V_s is anomalously large and negative, and possible origins of the effects observed are examined. In particular, the distance of closest approach, mutual salting-out associated with the large size of the ions and mutual salting-in arising from the hydrophobic character of the cations, are suggested as some of the reasons for the anomalies in the partial molal volume and activity coefficient behaviour which are exhibited by these salts.