  We study the evolution of phosphorous-bearing species in one-dimensional
C-shock models. We find that the abundances of P-bearing species depend
sensitively on the elemental abundance of P in the gas phase and on the
abundance of N atoms in the pre-shock gas. The observed abundance of PN and the
non-detection of PO towards L1157 B1 are reproduced in C-shock models with
shock velocity v=20km s-1 and pre-shock density n(H2) =10^4 - 10^5, if the
elemental abundance of P in the gas phase is 10^-9 and the N-atom abundance is
n(N)/nH - 10^-5 in the pre-shock gas. We also find that P-chemistry is
sensitive to O- and N-chemistry, because N atoms are destroyed mainly by OH and
NO. We identify the reactions of O-bearing and N-bearing species that
significantly affect P chemistry.
