Pyruvate decarboxylase (PDC) contains thiamine pyrophosphate (TPP) and Mg2+ as cofactors. 31P NMR studies with PDC in the presence of added Mn2+ reveal the pyrophosphate moiety of TPP to be a nonaccessible area for the external Mn2+ and thus proving the Mg-P-complex (taking part in the binding of the coenzyme to the protein) to be a nonaccessible area for the medium. Glyoxylic acid, acting as an inhibitor of PDC by forming a noncleavable bond with the catalytic center of TPP causes a steric immobilization of the coenzyme indicated by a line broadening of the pyrophosphate moiety.