Field desorption (FD) ionization is studied for the measurement of the molecular weight distribution of heavy petroleum, vacuum residua, and asphaltene derived from them. Hydrocarbon polymers, heavy petroleum distillation cuts, and chemical and solubility fractions of heavy petroleums were examined to determine the ionization efficiency. FD showed effective ionization of molecules of a wide molecular weight range (300-5000 Da) with different chemical compositions (saturates versus aromatics) and solubility (maltene versus asphaltene) properties. Electrospray ionization (ESI) was explored and compared with FD. Previous studies had shown that ESI is selective toward the ionization of polar molecules (mostly basic and acidic molecules) while FD is a more universal ionization method. The comparison revealed general agreement between the two techniques. Small variation in the molecular weight distribution of polar and nonpolar molecules was observed. ESI yields lower average molecular weights. The difference increases with the boiling point.