Blemishes rapidly developed on the surface of freshly poured structural concrete in Trinidad. The affected concrete was unsightly and suffered some loss of strength. Laboratory investigations showed that apparently inert natural sand and gravel aggregates used in the concrete were in fact coated or impregnated with an organic-iron complex which could be mobilized by cement alkalis to stain the concrete and retard the hydration of the surrounding cement. The reaction occurred when the total alkali content of the cement exceeded about 0.500y mass, expressed as Na~O equivalent. Lithologically similar aggregates from a neighbouring deposit were found to be predominantly non-reactive. When these were substituted for the original source, there was no recurrence of the problem. This paper describes the procedures which were used in the investigations, and discusses the character and occurrence of the alkali-reactive material. A simple test is suggested for the identification of aggregates which might react in this way. The authors are not aware of any published account of this reaction, although similar environmental factors might occur widely in tropical regions.