The comment by Nicholson (2011a) questions the "consistency" of the "definition" of the "biological end-member" used by Kaiser (2011a) in the calculation of oxygen gross production. "Biological end-member" refers to the relative oxygen isotope ratio difference between photosynthetic oxygen and Air-O₂ (abbreviated ¹⁷&delta;_P and ¹⁸&delta;_P for ¹⁷O/¹⁶O and ¹⁸O/¹⁶O, respectively). The comment claims that this leads to an overestimate of the discrepancy between previous studies and that the resulting gross production rates are "30% too high". Nicholson recognises the improved accuracy of Kaiser's direct calculation ("dual-delta") method compared to previous approximate approaches based on ¹⁷O excess (¹⁷&Delta;) and its simplicity compared to previous iterative calculation methods. Although he correctly points out that differences in the normalised gross production rate (<i>g</i>) are largely due to different input parameters used in Kaiser's "base case" and previous studies, he does not acknowledge Kaiser's observation that iterative and dual-delta calculation methods give exactly the same <i>g</i> for the same input parameters (disregarding kinetic isotope fractionation during air-sea exchange). The comment is based on misunderstandings with respect to the "base case" ¹⁷&delta;_P and ¹⁸&delta;_P values. Since direct measurements of ¹⁷&delta;_P and ¹⁸&delta;_Pdo not exist or have been lost, Kaiser constructed the "base case" in a way that was consistent and compatible with literature data. Nicholson showed that an alternative reconstruction of ¹⁷&delta;_P gives <i>g</i> values closer to previous studies. However, unlike Nicholson, we refrain from interpreting either reconstruction as a benchmark for the accuracy of <i>g</i>. A number of publications over the last 12 months have tried to establish which of these two reconstructions is more accurate. Nicholson draws on recently revised measurements of the relative ¹⁷O/¹⁶O difference between VSMOW and Air-O₂ (¹⁷&delta;_VSMOW; Barkan and Luz, 2011), together with new measurements of photosynthetic isotope fractionation, to support his comment. However, our own measurements disagree with these revised ¹⁷&delta;_VSMOW values. If scaled for differences in ¹⁸&delta;_VSMOW, they are actually in good agreement with the original data (Barkan and Luz, 2005) and support Kaiser's "base case" <i>g</i> values. The statement that Kaiser's <i>g</i> values are "30% too high" can therefore not be accepted, pending future work to reconcile different ¹⁷&delta;_VSMOW measurements. Nicholson also suggests that approximated calculations of gross production should be performed with a triple isotope excess defined as ¹⁷&Delta;^#&equiv; ln (1+¹⁷&delta;)–&lambda; ln(1+¹⁸&delta;), with &lambda; = &theta;_R = ln(1+¹⁷&varepsilon;_R ) / ln(1+¹⁸&varepsilon;_R). However, this only improves the approximation for certain ¹⁷&delta;_P and ¹⁸&delta;_P values, for certain net to gross production ratios (<i>f</i>) and for certain ratios of gross production to gross Air-O₂ invasion (<i>g</i>). In other cases, the approximated calculation based on ¹⁷&Delta;^&dagger; &equiv;¹⁷&delta; – &kappa; ¹⁸&delta; with &kappa; = &gamma;_R = ¹⁷&varepsilon;_R/¹⁸&varepsilon;_R (Kaiser, 2011a) gives more accurate results.