The interface response functions for rapid solidification of a non-dilute binary alloy were measured in the regime of partial solute trapping, where substantial discrepancies exist among predictions for the interfacial undercooling in various models. We used pulsed laser melting of Si-As on insulating substrates to enforce planar solidification spanning the velocity range 0.2–2 m/s. Nanosecond-resolution electrical measurements of the time-dependent melt depth and of the electrical resistivity of a buried Pt thin film thermometer permitted us to determine the solidification velocity and the temperature of the crystal/melt interface. With composition–depth profile measurements we also determined the nonequilibrium partition coefficient. The measured velocity-dependence of the interface temperature and partition coefficient are quantitatively consistent with the continuous growth model without solute drag of M. J. Aziz and T. Kaplan [Acta Metall. 36, 1335 (1988)] and are qualitatively and quantitatively inconsistent with all models exhibiting a significant solute drag effect. Elements of a potential explanation are proposed using the solute drag model of M. Hillert and B. Sundman [Acta Metall. 24, 731 (1976)] to investigate the origin of the solute drag effect in terms of irreversible processes occurring within a diffuse interface.