A high‐precision, and rapid on‐line method for oxygen isotope analysis of silver phosphate is presented. The technique uses high‐temperature elemental analyzer (EA)‐pyrolysis interfaced in continuous flow (CF) mode to an isotopic ratio mass spectrometer (IRMS). Calibration curves were generated by synthesizing silver phosphate with a 13‰ spread in δ¹⁸O values. Calibration materials were obtained by reacting dissolved potassium dihydrogen phosphate (KH₂PO₄) with water samples of various oxygen isotope compositions at 373 K. Validity of the method was tested by comparing the on‐line results with those obtained by classical off‐line sample preparation and dual inlet isotope measurement. In addition, silver phosphate precipitates were prepared from a collection of biogenic apatites with known δ¹⁸O values ranging from 12.8 to 29.9‰ (V–SMOW). Reproducibility of ± 0.2‰ was obtained by the EA‐Py‐CF‐IRMS method for sample sizes in the range 400–500 µg. Both natural and synthetic samples are remarkably well correlated with conventional ¹⁸O/¹⁶O determinations. Silver phosphate is a very stable material and easy to degas and, thus, could be considered as a good candidate to become a reference material for the determination of ¹⁸O/¹⁶O ratios of phosphate by high‐temperature pyrolysis. Copyright © 2006 John Wiley & Sons, Ltd.