A search for the J = 2-1 transition toward both hot and cold molecular gas is reported. SiO was observed toward all sources with kinetic temperatures greater than or equal to 30 K. No SiO was detected toward the dark clouds TMC-1, L134 N, and B335. The abundances of SiO relative to HCN and the SiO/H2 ratios are determined. A linear relation between the natural log of the SiO concentration and 1/Tk is found, suggesting that species formation involves a chemically specific process with an activation barrier of about 90 K. It is concluded that the formation of SiO is linked to the local gas kinetic temperature, rather than the oxygen abundance.