<p><strong>Abstract.</strong> The growing field of ocean acidification research is concerned with the investigation of organism responses to increasing <i>p</i>CO₂ values. One important approach in this context is culture work using seawater with adjusted CO₂ levels. As aqueous </i>p</i>CO₂ is difficult to measure directly in small-scale experiments, it is generally calculated from two other measured parameters of the carbonate system (often

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... </sub>, <i>C</i>_<i>T</i> or pH). Unfortunately, the overall uncertainties of measured and subsequently calculated values are often unknown. Especially under high <i>p</i>CO₂, this can become a severe problem with respect to the interpretation of physiological and ecological data. In the few datasets from ocean acidification research where all three of these parameters were measured, <i>p</i>CO₂ values calculated from <i>A</i>_<i>T</i> and <i>C</i>_<i>T</i> are typically about 30% lower (i.e. ~300 μatm at a target <i>p</i>CO₂ of 1000 μatm) than those calculated from <i>A</i>_<i>T</i> and pH or <i>C</i>_<i>T</i> and pH. This study presents and discusses these discrepancies as well as likely consequences for the ocean acidification community. Until this problem is solved, one has to consider that calculated parameters of the carbonate system (e.g. <i>p</i>CO₂, calcite saturation state) may not be comparable between studies, and that this may have important implications for the interpretation of CO₂ perturbation experiments.</p>