The importance of correcting for the thermal-lag effect in Sea-Bird CTD data is established for the strongly stratified waters of the Mediterranean thermocline in summer. Data collected with a SBE-25 CTD during an in situ experiment in the Balearic Sea in early summer are analysed. From the comparison between down and upcasts data, a large thermal-lag effect that contaminates the salinity data by errors up to 0.1 psu is diagnosed. A standard correction procedure is applied and results are discussed.