A total of 3,817 exhibits were scanned by Fourier transform infrared spectrometry. Five major groups were distinguished: (1) heroin free base; (2) heroin base with caffeine; (3) heroin base with paracetamol (acetaminophen); (4) heroin base containing both caffeine and paracetamol; and (5) a mixture of heroin base, heroin HCl and caffeine. In addition, 419 exhibits collected during a 2-month period were compared among themselves and categorized into 116 subgroups based on the similarity of their spectra. Of the total number of cases tested, 78 percent were user quantities. Results were tabulated on both a geographical and monthly basis. Differences were found in the distribution of additives between different geographical areas. The authors concluded that these comparison data alone are not sufficient to prove a connection between seizures or the existence of a common source, but can be useful when combined with an appropriate investigation. Figures, tables, references