The linear, unstable HC5P molecule has been detected for the first time in the pyrolysis products of phosphorus trichloride and toluene mixtures. Its rotational spectrum has been investigated in the millimeter-wave region (78-195 GHz) for the ground and v(11) = 1 excited state of both normal and deuterated species. Accurate values of rotational, centrifugal distortion and q(11) l-type doubling constants have been obtained. The experimental work was assisted by coupled-cluster single double triple [CCSD(T)] calculations, which provided accurate predictions for the equilibrium structure and the dipole moment of this new carbon chain, phosphorus bearing molecule. (C) 2003 American Institute of Physics.