<jats:p>The putative interspecific hybrid known as Eucalyptus brachyphylla C. Gardner, which is known to occur at two sites near Kalgoorlie, Western Australia, contains the non-terpenoid ester 4-methyl-2-pentyl acetate and the related alcohol 4-methylpentan-2-ol in its leaves. These compounds are otherwise known to occur in the leaves of only four Eucalyptus species and several putative interspecific hybrids. This, with the results of multivariate analyses of morphometric and oil composition data, adds considerable weight to;earlier suggestions that E. 'brachyphylla' is an hybrid of E. loxophleba Benth. and E. kruseana F. Muell. The presence at one site of two distinct chemotypes common to both parental species and to the hybrid plants is demonstrated and discussed.