Monomethyl malonates (IIIa-c and Va) were synthesized from Meldrum's acid (Ia) by treating with an excess of diazomethane in the corresponding alcohols and piperidine, respectively, in quantitative yields. In a similar manner, 5-p-chlorobenzyl Meldrum's acid (Ib) afforded the corresponding monomethyl malonate derivatives (IIIe, f, and Vb). The mechanism proposed for this reaction is described.