Phosphorus was determined in samples of glass from laboratory glassware, using a hydrofluoric digestion followed by colorimetry. Glass from such sources is shown to contain up to 0.002% of phosphorus. Experiments on storage of samples in glass bottles show that glassware may contribute significant phosphorus contamination to natural water samples, particularly seawater, as a result of partial dissolution of glass into the sample during storage. The release of phosphate from glassware into the sample is greater than the concentration expected from the P/Si ratio in glass and the silicate dissolution in the sample. This is attributed to the formation of a leached layer at the surface of the glass in contact with the water. Under the alkaline digestion conditions applied in this work for the total phosphorus determination, the dissolution of glass does not appear to be a source of measurable phosphate contamination. Nevertheless each analyst must satisfy himself that no phosphate contamination occurs under his particular working conditions with his own glassware.