The harmonic and anharmonic force fields of the title compounds have been calculated at the ab initio self-consistent-field level using effective core potentials and polarized double-zeta basis sets. Additional calculations for PH2F employ larger basis sets and include electron correlation. Many rovibrational constants are predicted theoretically. The infrared spectra generated from the ab initio data have guided the experimental identification of PH2F and PH2Cl in the gas phase. High resolution Fourier transform infrared spectra of these unstable molecules have been recorded for the first time. Rotational analyses for several bands are reported which provide-accurate ground state constants and a precise characterization of a number of vibrationally excited states. The accuracy of the ab initio predictions for PH2F and PH2Cl is evaluated by comparisons with these experimental data.