Anomalous doping behavior of the most commonly used n‐type dopants, Si and Sn, has been observed in InP grown by chemical beam epitaxy from trimethylindium and cracked phosphine. In the case of Si, although incorporation is initially facile, the doping level decreases in successive runs. It is proposed that this decrease is due to the formation of SiC in the dopant cell. The behavior of Sn is more complex. In the range of Sn cell temperatures from 600 to 750 °C, incorporation is proportional to the elemental Sn vapor pressure. However, at low cell temperatures, 175–350 °C, anomalously large incorporations of Sn are observed. We attribute this behavior to the formation of a volatile metalorganic precursor within the Sn cell, its transport to the substrate, and subsequent decomposition and incorporation.