A chemisorption process model has been developed and a sticking coefficient has been formulated by taking the surface migration of physisorbed particles into account. This model incorporates Langmuir’s model for use when the migration barrier is infinite, and the standard precursor model for use when dealing with a bare surface. The migration of incident particles was found to make the sticking coefficient independent of the surface coverage at low temperatures (approximate to 100 K). At higher temperatures, the migration appears to be a hidden parameter in the chemisorption process. The effect of migration can probably be confirmed, even at room temperature, by analyzing the chemisorption using free radicals.