The microstructural changes inside the lamellar structure of the cast and aged alloy ZA27 were studied using TEM, XRD and SEM techniques. Using TEM, the network of transitional phase η m ′ was determined to be of an fcc crystal structure inside the lamellae η phase during ageing at 150 °C. The mechanism of the decomposition of the η phase lamellae can be summarized as follows: η→η m ′ +η′→α+η. The adjacent co-existence of the ε phase and the T′ phase inside the α phase lamellae confirmed that a four phase transformation, α+ε→T′+η, had occurred during the prolonged ageing.