ATP in dilute solutions prepared for use as internal standards in the bioluminescence assay is gradually degraded if not continually kept at 0°C or below. The example in this note indicates over 160% recovery of ATP in samples due to the decreased ATP concentration in the internal standard stock solution over a period of 90 min. The degradation of ATP may be of concern in context of ATP‐analyzers with ATP‐injection systems unless precautions are undertaken to stabilize dilute ATP standards.