Carboxyfullerene synthesis involves two reactions addition of esters of malonic acid to give mixtures of adducts which are separated by chromatography and (2) conversion of esters to acids. We have prepared esters using dimethyl, diethyl and di-t-butyl malonates and confirmed that the product distribution is highly dependent on the size of the R group, yield of dimethyl C3> diethyl C3 >> di-t-butyl C3. The mass spectra (FAB+) confirmed the number of additions but the spectra were nearly identical for all isomers. Conversion of C3 esters (methyl or ethyl) to acid gave mixtures of acids which all retained the C3 symmetry. The three major components were identified as hexa acid and two isomeric penta acids. Penta esters were found in the unhydrolyzed portion of an incomplete reaction mixture, suggesting that the carboxyl was lost prior to hydrolysis. A variety of mass spectrometric techniques are used to characterize products and byproducts.