Radiative cooling by seeded impurities is a way to reduce the power load on divertor targets. Impurity-seeding experiments carried out in the JET MkI, MkIIA and MkII GB divertors are discussed, in particular in terms of their radiative properties. A reassessment of the radiation levels of about 235 discharges (seeded predominantly with nitrogen or neon) leads to the conclusion that radiation levels in some impurity-seeded plasmas in MkI and MkIIA have been underestimated. The techniques applied to obtain improved estimates of the total radiated power and radiated power in the divertor region are discussed and it is shown that weighted summation is unreliable for impurity-seeded discharges in JET. The consequences of the reassessment on the interpretation of impurity-seeded plasmas is limited. However, the understanding of the power balance and the fit to Matthews' scaling law for Zeff are improved.