The interface when switching from AlAs to GaAs during solid source molecular-beam epitaxial growth is investigated. The growth conditions for the AlAs layers were kept constant except for the As overpressure. Using a valved As cracker cell, we varied the V/III flux ratio from similar to 5.0 to similar to 25.0. Cross-sectional transmission electron microscopy, photoluminescence spectroscopy, and reflectivity measurements from distributed Bragg reflectors indicate that the material quality tends to improve with increasing dimeric As overpressure. Using secondary ion mass spectroscopy, it is shown that the rough interfaces are due to oxygen accumulation at the AlAs growth front. It is believed that arsenic forms an oxide with the oxygen on the AlAs surface, which is subsequently desorbed away at typical growth temperatures. For samples grown at higher overpressures, there is more arsenic present to remove the oxygen thereby resulting in a smoother interface