Hydroxymethoxycarbene, CH3OCOH, was produced via pyrolysis of monomethyl oxalate and subsequently isolated in He-4 nanodroplets. Infrared laser spectroscopy reveals two rotationally resolved a,b-hybrid bands in the OH-stretch region, which are assigned to trans,trans- and cis,trans-rotamers. Stark spectroscopy of the trans,trans-OH stretch band provides the alpha-axis inertial component of the dipole moment, namely) mu(a)= 0.62(7) D. The computed equilibrium dipole moment agrees well with the expectation value determined from experiment, consistent with a semi-rigid CH3 OCOH backbone computed via a potential energy scan at the B3LYP/cc-pVTZ level of theory, which reveals substantial conformer interconversion barriers of approximate to 17 kcal/mol. (C) 2015 Elsevier B.V. All rights reserved.