Phosphorus trichloride initially physically adsorbs on silica at room temperature via an interaction with surface silanol groups. Following prolonged contact over several hours,it dissociates to yield the chemisorbed SiOP-containing species SiOPCl2, SiOP=O(H,OH), and(SiO)2P=O(H). The last two of these species can also be produced by impregnation of silica with phosphorous acid, H3PO3, from aqueous solution. Phosphoryl chloride, O=PCl3, also physically adsorbs on silanols, but the interaction is stronger than that which occurs with PCl3, and no further reaction occurs at room temperature. Some anomalies previously reported in the literature from earlier studies of PCl3 adsorption on silica have been shown to be due to OPCl3 which can be present as a very low level (1-2 %) impurity in commerially available PCl3.