  We present a comprehensive examination of interstellar P and Cl abundances
based on an analysis of archival spectra acquired with the Space Telescope
Imaging Spectrograph of the Hubble Space Telescope and the Far Ultraviolet
Spectroscopic Explorer. Column densities of P II, Cl I, and Cl II are
determined for a combined sample of 107 sight lines probing diffuse atomic and
molecular gas in the local Galactic interstellar medium (ISM). We reevaluate
the nearly linear relationship between the column densities of Cl I and H$_2$,
which arises from the rapid conversion of Cl$^+$ to Cl$^0$ in regions where
H$_2$ is abundant. Using the observed total gas-phase P and Cl abundances, we
derive depletion parameters for these elements, adopting the methodology of
Jenkins. We find that both P and Cl are essentially undepleted along sight
lines showing the lowest overall depletions. Increasingly severe depletions of
P are seen along molecule-rich sight lines. In contrast, gas-phase Cl
abundances show no systematic variation with molecular hydrogen fraction.
However, enhanced Cl (and P) depletion rates are found for a subset of sight
lines showing elevated levels of Cl ionization. An analysis of neutral chlorine
fractions yields estimates for the amount of atomic hydrogen associated with
the H$_2$-bearing gas in each direction. These results indicate that the
molecular fraction in the H$_2$-bearing gas is at least 10% for all sight lines
with $\log N({\rm H}_2)\gtrsim18$ and that the gas is essentially fully
molecular at $\log N({\rm H}_2)\approx21$.
