Metastable structures formed in ZrMo alloys on quenching and following high pressure application were studied by transmission electron microscopy and X-ray phase analysis. New structures consisting of a mixture of α phase and ω phase originating in α [i.e. ω(α)] were observed. Complex structures form in quenched Zr alloys with 1.5-2.5 at.% Mo which contain α,β and ω originating in both α and β. The maximum possible number of the co particle orientations (ω-variants) in a sample is seven: three for ω(α) and four for ω(β). High pressure application to the quenched structures gives rise to the α →ω(α) and β→ ω(β) transformation. Pressure induced ω phase can be retained at atmosphere pressure upon unloading. The genesis of various metastable structures is proposed by consideration of metastable phase equilibria existing in the ZrMo system.