There is good evidence that a phenomenon resembling Gibbsian segregation, i.e. involving a surface composition spike, occurs when alloys are bombarded either in the absence or presence of oxygen. The existence of such segregation means that the non-segregating species will be present in, and be emitted from, atom layer two to an untypical extent. The present work explores the consequences of this unusual situation for secondary ion formation. In particular, emission from atom layer two is shown to be numerically significant for a dilute non-segregating species but the sense of the effect, i.e. which species is non-segregating, is sometimes reported incorrectly. The non-segregating species will show characteristic differences in emission, including preferential emission near the target normal and a slightly narrower energy distribution. There is a possibility that the matrix effect associated with O2+ bombardment or O2 flooding in part arises because a non-segregating species passes through a greater thickness of target in escaping. © 1986.