  A thorough understanding of native oxides is essential for designing
semiconductor devices. Here we report a study of the rate and mechanisms of
spontaneous oxidation of bulk single crystals of ZrS$_x$Se$_{2-x}$ alloys and
MoS$_2$. ZrS$_x$Se$_{2-x}$ alloys oxidize rapidly, and the oxidation rate
increases with Se content. Oxidation of basal surfaces is initiated by
favorable O$_2$ adsorption and proceeds by a mechanism of Zr-O bond switching,
that collapses the van der Waals gaps, and is facilitated by progressive redox
transitions of the chalcogen. The rate-limiting process is the formation and
out-diffusion of SO$_2$. In contrast, MoS$_2$ basal surfaces are stable due to
unfavorable oxygen adsorption. Our results provide insight and quantitative
guidance for designing and processing semiconductor devices based on
ZrS$_x$Se$_{2-x}$ and MoS$_2$, and identify the atomistic-scale mechanisms of
bonding and phase transformations in layered materials with competing anions.
