For example, in the case of non-dissociative adsorption, the rate of the forward process is given by

$$\frac{\mathrm{d}\theta}{\mathrm{d}t} = kCN(1−\theta)$$

where $C$ is the concentration, $N$ is the number of surface sites, and $\theta$ is the fraction of filled sites.

The reverse reaction is given by

$$\frac{\mathrm{d}\theta}{\mathrm{d}t} = −kN\theta.$$

Hence, it seems like the first reaction is second-order, while the second is first order. This seems strange to me.