a b s t r a c t An 18-month time series of temperature collected in the vicinity of the Guadalquivir estuary mouth has been analyzed to investigate tidal signals in the temperature records. Two sources of similar importance are acting jointly to produce these signals, the gravitational tide generating potential that acts through the tidal currents induced by the vertical tide at the estuary's mouth and the radiational potential whose origin is the sun's radiation. The most important signal is the radiational diurnal S1, which exhibits a seasonal modulation. The next one in importance is M2, introduced via the advective term, which shows semiannual modulation due to the change of sign of the horizontal temperature gradient. The S2 constituent is a mixture of radiational (via the first harmonic of S1) and gravitational contributions, the former being about twice the latter. The harmonic constants of all these constituents are time-dependent so that caution is needed to interpret results deduced from time series of few months.