Combined computational-experimental studies were carried out to parallel two mechanistic models for tetrahydropyranylation of alcohols catalyzed by Schreiner's thiourea. The results challenge the common mechanistic view that the catalytic effect is related to stabilizing double hydrogen-bonding interactions between the thiourea and the alcohol, which promote the attack on 3,4-dihydro-2H-pyran (DHP; HB mechanism). In the alternative mechanism we propose, thiourea acts as a Brønsted acid, protonating DHP to form an oxacarbenium ion, which reacts with the alcohol (BA mechanism). Computations point to clear preference of transition states associated with the BA mechanism and, accordingly, predict similar catalytic activity for Nmethylated thiourea and thiouracil. These predictions are confirmed experimentally. Reactions with deuterated alcohols yield both syn and anti products, providing further support for the Brønsted acid mechanism.