X-ray photoelectron spectroscopy (XPS) measurements were carried out on a strontium pyroniobate (Sr2Nb2O7) powder sample, which was synthesized using standard solid-state method. The binding energy (BE) differences between the O 1s and cation core levels, Delta(O-Nb) = BE(O 1s)-BE(Nb 3d(5/2)) and Delta(O-Sr) = BE(O 1s)-BE(Sr 3d(5/2)), were used to characterize the valence electron transfer on the formation of the Nb-O and Sr-O bonds. The chemical bonding effects were considered on the basis of our XPS results for Sr2Nb2O7 and earlier published structural and XPS data for other Sr- or Nb-containing oxide compounds. The new data point for Sr2Nb2O7 is consistent with the previously derived relationship for a set of Nb5+ -niobates that Delta (O-Nb) increases with increasing mean Nb-O bond distance, L(Nb-O). A new empirical relationship between Delta(O-Sr) and L(Sr-O) was also obtained. Interestingly, the correlation between Delta(O-Sr) and L(Sr-O) was found to differ from that between Delta(O-Nb) and L(Nb-O). Possible cause for the difference is discussed. (c) 2008 Elsevier Inc. All rights reserved.