Phytochelatins and glutathione (reduced (GSH) and oxidised (GSSG)) are important intracellular lig-ands involved in metal sequestration and detoxification in algae. Intracellular ratios of GSH:GSSG aresensitive indicators of metal stress in algae, and like phytochelatin production are influenced by metalspeciation, concentration, exposure time and the biological species. This study investigated the effectof copper exposure on phytochelatin and glutathione content in two marine diatoms Phaeodactylumtricornutum and Ceratoneis closterium at various time intervals between 0.5 and 72 h. Liberation of cel-lular glutathione and phytochelatins was optimised using freeze/thaw cycles and chemical extraction,respectively. Extracted phytochelatins were derivatised (by fluorescent tagging of thiol compounds), sep-arated and quantified using HPLC with fluorescence detection. Glutathione ratios were determined usinga commercially available kit, which uses the enzyme glutathione reductase to measure total and oxidisedglutathione.Despite similarities in size and shape between the two diatoms, differences in internalised copper, phy-tochelatin production (both chain length and quantity) and reduced glutathione concentrations wereobserved. P. tricornutum maintained reduced glutathione at between 58 and 80% of total glutathionelevels at all time points, which would indicate low cellular stress. In C. closterium reduced glutathioneconstituted <10% of total glutathione after 48 h. P. tricornutum also produced more phytochelatins andphytochelatins of longer chain length than C. closterium despite the latter species internalising signifi-cantly more copper.