Rotationally resolved infrared spectra are reported for the linear and T-shaped isomers of acetylene-hydrogen cyanide formed in helium nanodroplets. The results for the latter show that the overall rotation ((B + C)/2) is slowed by the helium, while rotation about the A-axis is only weakly affected. A comparison of the relative abundances of the two isomers formed in helium and in a gas phase free jet expansion provides insights into the nature of the growth in helium.