The compatibility between isoniazid (INH) and pharmaceutical excipients was studied using thermal analysis (DSC and TG/DTG) and vibrational spectroscopy (FTIR). Modifications in the peak shape, peak onset or peak maximum were verified using thermal analysis, especially DSC, in the physical mixtures. FTIR spectra showed good correlation between INH and most of excipients. Lactose and sodium metabisulfite were found to exhibit interactions with isoniazid.