The interference of the organophosphorus acids, 1-hydroxyethane-l,l-bisphosphonic acid, aminotris(methylenephosphonic acid), ethylenediaminetetrakis( methylenephosphonic acid) and hexamethylenediaminetetrakis(methylenephosphonic acid) on the determination of eighteen metal ions by flame atomic absorption spectrometry is reported. Comparisons with the effect of orthophosphoric acid reveal similarities and distinct differences in their interfering effects. In the air/acetylene flame, depressive interferences are attributed to the formation of phosphates, M,(PO,),, or hydroxyapatite-like compounds, M,(OH)(PO,),, in the flame aerosol particles for Mg, Ca, Sr, Ba, Mn, Co and Ni. Iron(II1) and chromium( III) appear to form stable oxide phosphates, M,O, . MPO, or M,O, .MPO,. Evidence for the formation of stable molybdenum carbides, MoC and MoC,, is also presented. In the nitrous oxide/acetylene flame, serious interferences persisted only for molybdenum but were eliminated by the addition of sodium sulphate. (OHl20PCH2 CH2PO(OH)2 \ N-CH2-CH2-N' \ CH2PO(OH12 CL CH2 'N , CH2PO(OH12 \2 C;H2 ' CH2PO(OH)2 /CH2 CH \2 , N , CH2PO(OH)2 CH2 ' CH2PO(OH)2