An antacid composition, capable of maintaining artificial gastric juice at a pH of from 3 to 5 for at least one hour, comprises magnesium and/or calcium aluminium hydroxy carbonate having an Al2O3 : MO (where M is magnesium and/or calcium) molecular weight ratio of from 4 : 1 to 1 : 10.  The composition is prepared by forming a coprecipitate complex of magnesium and/or calcium and aluminium from a common aqueous solution of magnesium and/or calcium, aluminium and hydroxide ions, while maintaining the solution pH at from 9 to 10, and reacting the coprecipitate complex with carbon dioxide at a solution pH of from 6 to 10.  The carbon dioxide is preferably added as the gas.  The carbonated coprecipitate complex may be separated from the reaction solution, washed to remove soluble salts and subsequently dried.  The preferred method of preparation is to blend an aqueous solution of an alkali metal aluminate with an aqueous solution of a water-soluble magnesium and/or calcium salt by adding the two solutions to water at a rate and in proportions to maintain the pH of the resulting mixture at from 9 to 10; the pH may be brought to within this range by addition of alkali metal carbonate or alkali metal hydroxide.  The coprecipitate may be aged before and/or after carbonation. The complex may be simultaneously precipitated and reacted with carbon dioxide.  The composition may be in the form of an aqueous suspension, or a gel which does not thicken on ageing, or a dry powder produced by spray-drying the suspension or by air-drying the gel. The composition may be tableted, using the customary incipients, for example a polyol such as mannitol, a sweetener such as dextrose, and a lubricant such as magnesium stearate.  The prior art is discussed in detail in the Specification, numerous references being given.ALSO:An antacid composition, capable of maintaining artificial gastric juice at a pH of from 3 to 5 for at least one hour, comprises magnesium and/or calcium aluminium hydroxy carbonate having an Al2O3:MO (where M is magnesium and/or calcium) molecular weight ratio of from 4:1 to 1:10.  The composition is prepared by forming a coprecipitate complex of magnesium and/or calcium and aluminium from a common aqueous solution of magnesium and/or calcium, aluminium and hydroxide ions, while maintaining the solution pH at from 9 to 10, and reacting the coprecipitate complex with carbon dioxide at a solution pH of from 6 to 10.  The carbon dioxide is preferably added as the gas.  The carbonated coprecipitate complex may be separated from the reaction solution, washed to remove soluble salts and subsequently dried.  The preferred method of preparation is to blend an aqueous solution of an alkali metal aluminate with an aqueous solution of a water-soluble magnesium and/or calcium salt by adding the two solutions to water at a rate and in proportions to maintain the pH of the resulting mixture at from 9 to 10; the pH may be brought to within this range by addition of alkali metal carbonate or alkali metal hydroxide.  The co-precipitate may be aged before and/or after carbonation.  The complex may be simultaneously precipitated and reacted with carbon dioxide.  The composition may be in the form of an aqueous suspension, or a gel which does not thicken on aging, or a dry powder produced by spray-drying the suspension or by air-drying the gel; alternatively, the composition may be tableted.  The prior art is discussed in detail in the Specification, numerous references being given.