In a method of synthesizing a silicoaluminophosphate molecular sieve comprising at least one intergrown phase of an AEI framework type material and a CHA framework type material, a first synthesis mixture is prepared comprising water and sources of phosphorus, aluminum and optionally silicon. The first synthesis mixture is then heated under agitation to a first temperature to form an intermediate product mixture containing a silicoaluminophosphate or aluminophosphate precursor material. The intermediate product mixture is then cooled and stored at a second temperature lower than the first temperature, whereafter a second synthesis mixture is prepared comprising at least part of the intermediate product mixture and at least one organic directing agent. The second synthesis mixture is heated to a third temperature higher than the second temperature to convert at least part of the precursor material into the desired molecular sieve and the molecular sieve is recovered.