The existence and nature of dislocation loops (DL) and the heterogeneous nucleation of precipitates at them are studied in three Al‐(3.0, 12 and 15 at%) Zn as well as in two AlZn(2x)‐Mg(x) alloys (x = 1.0 and 1.5at%) using TEM. The most important results are: (i) Not only in the AlZn, but also in the AlZnMg alloys DL are observed, although in the latter, according to the literature their formation is not expected, (ii) The heterogeneous nucleation of precipitates is proceeding in quite a different manner in both alloy systems. The reasons supposed are discussed.