Materials suitable for use in non-linear optical devices comprise morpholinium or thiomorpholinium salts of hydroxy-substituted aliphatic or aromatic carboxylic acids selected from tartaric acid and hydroxy-substituted benzoic and cinnamic acids, which salts have a non-centrosymmetric crystalline form. Preferred salts include morpholinium 3-hydroxybenzoate, morpholinium 4-hydroxybenzoate, morpholinium 3,5-dihydroxybenzoate, morpholinium 3-fluoro-4-hydroxybenzoate, dimorpholinium 2,3,5,6-tetrafluoro-4-hydroxybenzoate, morpholinium 4-(4-hydroxyphenyl)benzoate, morpholinium tartrate, morpholinium 4-hydroxycinnamate, thiomorpholinium 4-hydroxy-benzoate and thiomorpholinium 3,5-hydroxybenzoate.