A fast, personal-computer based method of estimating molecular weights of organic compounds from low resolution mass spectra has been thoroughly evaluated. The method is based on a rule-based pattern recognition/expert system approach which uses empirical linear corrections which are iteratively applied to two mass spectral features to yield estimates. This technique has been extensively evaluated with 400 spectra of volatile and nonvolatile compounds of environmental and pharmaceutical interest and with 31378 high quality NIST reference spectra of compounds of molecular weight 30-500. For both sets of evaluation spectra the median and average absolute deviations were 1.5-2.0 and 13-17 daltons, respectively. Median errors for spectra with the molecular ion present were ca. twenty times lower than those without the molecular ion. The present system can rapidly produce molecular weight estimates of a wide variety of compounds with median absolute errors of 2 (average 15) daltons. Results with the 106 toxic and related training compounds show a median and average absolute deviation of 0 and 0.6 daltons.