van der Waals complexes consisting of a single OCS molecule with either a single para-H-2, normal H-2, ortho-D-2, or a HD molecule have been prepared inside large ultracold (0.37 K) He-4 droplets and even colder (0.15 K) mixed He-4/He-3 droplets. Their infrared (nu=2061 cm(-1)) spectra were measured using diode laser droplet beam depletion spectroscopy. The well-resolved rotational structure with up to about 18 lines could be very well fitted with the Hamiltonian of a free asymmetric top, with rotational constants which were found to be between 3 and 9 times smaller than for the corresponding free complexes. As recently reported [S. Grebenev , J. Chem. Phys. 114, 617 (2001)] the structure of the complex, obtained from the change in the moments of inertia for the three isotopomers, is in very good agreement with ab initio calculations and the high-resolution infrared spectra of the free complexes. These results provide new insight into the interaction of impurity molecules with the superfluid helium environment. (C) 2003 American Institute of Physics.