Interaction of phosphate moiety possessing compounds with sapphyrin was studied using open-tubular electrochromatography in sapphyrin-coated capillaries. It was revealed that phenylthiohydantoin (PTH)-phosphoserine and PTH-phosphothreonine exert such a strong interaction that they can not be eluted from the sapphyrin-coated capillary even at prolonged run times (70 min). Nucleoside polyphosphates show generally strong interaction (but weaker than the above mentioned serine and threonine derivatives) no matter whether they possess one or two bases. Also the number of phosphate residues present in nucleoside polyphosphates tested plays a secondary role only. p-Aminobenzylphosphoric (p-ABPA) acid exhibited an unexpected behavior. It was retained more in the phosphate containing buffer than in borate-acetate. This appears to indicate that other than complexing of the phosphate moiety may be involved in the interaction. As no such effects were observed with the PTH-derivatives of serine and threonine it was concluded that additional interaction (if involved) depends on the nature of the organic part of the molecule.