Calcium aluiiiiliiurii liydroxitlos were coprccipitatcd from cliffcrciit, iliixod metal cation solutioiis -at total C , = 0. I M arid Ca/AI, ratios from I to 4 -with sodium hydroxide solution at ambient temperature. The coprecipitations were monitored by potentiomotric (pH) titration arid the final coprecipitate compositions were examined by chemical analysis, infra-red spectrophotometry and thermal analysis Generally, microcrystalline aluminium hydroxide was first precipitated a t p H about 4; this then redissolved on further addition of sodium hydroxide to form hydroxoalumiiiate anion and polyaniori and ca1ciu.m aluminium hydroxide coprecipitates were formed continuously at p k l s from about 9 to above 12. Their compositions were similar to the calciiirn hydroxoalirrninato Iiydrates formed by direct precipitation from high p H sodium hydroxoaluminato soliitioiis.-A t Ca/AI, ratio-= 1, tho rnain phase was-probably Ca(H,O)h (AI (