The application of the in situ measurement system 'KATERINA' for monitoring of radon progenies in submarine groundwater discharge (SGD) was investigated at different locations in the Mediterranean Sea (Chalkida, Stoupa, Korfos and Cabbé). At Chalkida and Stoupa radon progenies concentration exhibited almost constant values of 1.2 ± 0.1 and 2.5 ± 0.2 Bq l(-1), respectively. At Korfos these activities ranged between 1.4 ± 0.1 and 2.3 ± 0.2 Bq l(-1) exhibiting inverse relationship with salinity. At Cabbé the in situ measured data were compared with radon measurements obtained by liquid scintillation counter. The system also resolved radon progeny variations of SGD on time scales above 1 h. The radioactivity levels of radon progenies from all sites were found considerably lower (approximately 2 orders of magnitude) than the commonly accepted limits for radon in drinking water.