A phase transition golf ball comprises a phase transition material. The phase transition material may optionally include a microwave susceptor or an induction susceptor. The phase transition material preferably comprises an ethylene acid copolymer, or an ionomer of an ethylene acid copolymer. The performance of the phase transition golf ball, for example its hardness or compression, is adjusted by inducing a complete or partial phase transition in the phase transition material. The extent of the adjustment in performance is correlated with the extent of the phase transition. Preferably, the phase transition is reversible and repeatable and takes place at temperatures that might be achieved through the use of common household appliances. Also preferably, the phase transition material returns to its original state over an extended period, for example hours or days.