Deep levels have been characterized in Si-doped and undoped InAlAs layers lattice matched to InP. At ambient pressure and low temperature, the Schottky junction capacitance shows a small persistent photocapacitance effect, related to photoionization thresholds at 0.6 and 1.1 eV. Deep-level transient spectroscopy spectra show a dominant, low-density electron trap at around 320 K, with an emission energy very dependent on the bias conditions and the hydrostatic pressure (0.5 to 0.9 eV). This is attributed to an interaction of a high density of interface states with the trap emission process. Under hydrostatic pressure, neither the PPC per cent nor the DLTS spectra shape changes. Photoluminescence spectra in both Si-doped and undoped InAlAs layers show two weak peaks at 0.80 and 0.95 eV, that the authors suggest are due to a native defect involving an As antisite. They conclude that the detected traps have no relation with the DX centres.