Observation indicates that the expansion of the Universe is accelerating and favours a dynamical cosmological constant, Λ(t). We consider the possibility that this is due to a scalar field which has undergone a very recent phase transition. We study a simple class of model, corresponding to a φ4 potential with a time-dependent mass squared term. For the models considered the phase transition occurs at a red shift z⩽1.2. The evolution of the equation of state ωφ and energy density ρφ with time is distinct from existing dynamical Λ models based on slowly rolling fields, with ωφ and ρφ rapidly changing in a characteristic way following the transition. The φ energy density is composed of a time-dependent vacuum energy and coherently oscillating condensate component with a negative pressure. The condensate component will typically collapse to form non-topological soliton lumps, ‘φ-axitons’, which smoothly populate the Universe.