A measurement station has been built for the non-destructive investigation of burnt fuel rod segments through high-resolution gamma spectrometry. Four UO 2 pressurised water reactor fuel rod segments with different burnup levels between 50 and >100GWd/t and ⩽10 year cooling time have been experimentally characterised using gamma-ray spectrometry to determine 134 Cs, 137 Cs and 154 Eu and their corresponding concentration ratios. Experimental errors of ∼2% (1σ) for the 134 Cs/ 137 Cs ratio were obtained for most of the segments. In parallel, pin cell depletion calculations have been performed for each segment using the deterministic code CASMO-4. Measured and calculated ratios have then been compared with the purpose of deriving and validating pin-averaged single-ratio burnup indicators for very high burnups. It is shown that the 134 Cs/ 137 Cs ratio, frequently used as a burnup monitor, is considerably less precise for values exceeding 50GWd/t; discrepancies of ∼16% are found between measured and calculated values, increasing with burnup up to ∼23%. The ratios built with the 154 Eu concentration show even much larger discrepancies, essentially because this isotope is rather poorly predicted as revealed by just using different basic cross section data.