The interaction of phosphine (PH3) with the Si(001) surface has been investigated with scanning tunneling microscopy. At 300 K, PH3 adsorbs molecularly on top of a single Si=Si dimer with the phosphorus atom in a pentacoordinate configuration. Phosphine adsorption is accompanied by occasional ejection of silicon atoms from the substrate onto the surface, creating surface vacancies and Si adatoms. Si atoms are preferentially ejected from defect sites. At high coverage, adsorbed PH3 molecules order into small regions of c(4X2) symmetry, and the ejected silicon nucleates into small dimer strings. The long-range order of the molecular PH3 is disrupted by domain boundaries and the ejected silicon. Annealing PH3-saturated surfaces produces further ejection of silicon onto the surface, resulting in large silicon islands covering approximately 20% of the surface.