New measurements have been made on the hydrogen-calomelcell at 25°C containing hydrochloric acid solutions of molality less than O·1. Two designs of calomel electrode compartment were used and a somewhat greater amount of calomel skin than that recommended by Hills and Ives, 1 but much less than that used for the classical form of electrode, was employed with advantage. Parallel measurements have been made on the hydrogen-silver/silver-chloride cell. Recalculation and a statistical comparison of all recently published data on both cells have been carried out. The analysis reveals small but statistically significant differences between activity-coefficient values derived from the so-called “first-day calomel electrode” of Hills and Ives and the later “second-day electrode”, which is in equilibrium with mercuric species in solution. The difference between the two calomel electrodes is not therefore just one of standard potential. The data on the silver/silver-chloride cells yield a set of activity-coefficient values intermediate between those from the “two” calomel electrodes but in closer agreement with Hills and Ives' first-day values if the results for molalities are less than 0·01, where other effects may be operative, are excluded. Best results are presented for the activity coefficient of hydrochloric acid at 25°C.