We report on the interference that hydrogen sulfide can generate on the measurement of phosphorus in seawater with the Murphy & Riley method. Free sulfide can be found in a variety of saline systems, from partially closed fjords to eutrophic estuaries, river saline wedges, saline lagoons, etc. in concentrations ranging from 0.02 up to 10 mM and, thus, may be present in water samples without the analysts suspicion. We tested the effects of sulfide concentrations ranging from 0.005 up to 0.7 mM on the measurement of phosphorus concentrations from 10 mgP to 1.5 mgP-PO4 l-1. We characterized spectrophotometrically, the product and the dynamics of the interference and we provide a simple method to detect the interference "a posteriori" by computing the ratio between the absorbances at 350 and 690 nm. If sulfide has been measured in parallel, the absorbances, even those that have had high interference, can be corrected to values without the interference with the equations that we provide. We propose the routine use of this method whenever dealing with samples that could have some sulfide in solution.