Rotationally resolved infrared spectra are reported for the T-shaped acetylene-HF complex in helium nanodroplets, corresponding to excitation of both the H-F and asymmetric C-H stretches. In comparison to the isolated complex, rotation about the A-axis is relatively unhindered by the helium, while overall rotation is slower by a factor of approximately two. An ab initio potential surface is also reported for this system. Although the bent complex, with acetylene acting as a hydrogen donor, is determined to be a local minimum, only the T-shaped isomer is experimentally observed, indicating that the barrier is too small to prevent the metastable bent structure from rearranging to the global minimum. (C) 2003 Elsevier B.V. All rights reserved.