The semi-stable HCP molecule has been produced by gas phase copyrolysis of phosphine and chloroform. The three rotational transitions J = 2 <-1, 3 <- 2, and 4 <- 3 have been recorded with sub-Doppler resolution using the Lamb-dip technique. The hyperfine structure due to the magnetic coupling between rotation and phosphorus nuclear spin has been resolved, yielding the value of the phosphorus spin-rotation constant, C(P) = 43.64 +/- 0.15 kHz, from which the corresponding absolute magnetic shielding has been derived: sigma(P) = 373 +/- 2 ppm. Theoretical values of C(P) and sigma(P) have also been calculated for HCP and NP at the CCSD(T) level of theory. (c) 2005 Elsevier B.V. All rights reserved.