K-A ages from a variety of alkaline rocks, carbonatites (sövites) and alnöites from the Alnö complex range between 370 and 605 m.y. but the most meaningful ages concentrate between 546 and 578 m.y. Most nephelinites, phonolites and alkali trachytes that occur in a dike complex surrounding the main intrusion give a middle Cambrian RbSr whole-rock isochron age of 553±6 m.y. (2σ). Some samples (deleted from the age regression) were contaminated by radiogenic strontium. Plutonic pyroxenites, ijolites and nepheline syenites that form most of the core of the intrusion formed at about the same time and from the same parental magma as the dike rocks. Sövites and alnöites that crosscut the dike and core rocks did not intrude significantly later and may have formed from the same parent magma or mantle source. Many samples show evidence of either mixing or isotopic exchange of the magma with the country rocks during intrusion (fenitisation?) or open system behavior after crystallization. These processes resulted in some scatter of the data points about the best-fit RbSr isochron lines and in some anomalous K-A ages. Fenites have higher Rb/Sr and 87Sr/86Sr ratios than the alkaline rocks, making it unlikely that the latter were the remobilized products of extreme fenitization, as suggested by von Eckermann (1948). The Alnö intrusion is about the same age as the Fen complex of southern Norway, but is significantly older or younger than many other apparently similar intrusions in Scandinavia.