Abstract: Amechanism of clevage-rate-determining-step in the mass spectra of furfurol, furylacrylic acid, dimethylfuran, thiophene-2-aldehyde, and pyrrole-2-aldehydc is proposed. Various quantum parameters of molecular ions together with their fra-ments of these compounds are calculated by CNDO/2 method. The peak-heights of the mass spectra can be quantitatively interpreted. This method may be extended to heterocyclic conjugated compounds.