Computer process modeling has become an important tool in the development of new thin-film processes. Previously, we presented a model that successfully describes the complex behavior of the reactive sputtering of a single-element target. The model enables one to predict, e.g., the hysteresis effect and the composition of the deposited film. There exists, however, a demand for developing reliable reactive-sputtering processes for more complicated materials. This calls for an extended reactive-sputtering model including more than one target element. In this article, we will for the first time present an extension of our previous model now taking the effect of multicomponent targets into consideration. The transition from metallic- to compound-sputtering mode normally occurs at different levels for different single-element targets. The new extended reactive-sputtering model predicts the transition from metallic to compound mode during multicomponent reactive sputtering of thin films. The results indicate that under normal operating conditions, only one transition region exists irrespective of the number of target elements involved in the process. This is experimentally verified for the process of reactive cosputtering of Al+Ti in an Ar/O2 atmosphere. We will also point out the complex composition behavior of the reactive-cosputtering process as compared with reactive sputtering from an alloy target.