Abstract: A golf ball includes an inner core layer including a thermoplastic highly-neutralized ionomer formed from a copolymer of ethylene and an ?,?-unsaturated carboxylic acid, an organic acid or salt thereof, and sufficient cation source to neutralize the acid groups of the copolymer by 80% or greater. The inner core has a geometric center hardness and a surface hardness to define a first hardness gradient. An outer core layer is disposed about the inner core and is formed from a homogenous thermoset composition. The outer core layer has an interior hardness and an outer surface hardness to define a second hardness gradient. An inner cover layer is disposed about outer core layer and an outer cover layer is disposed about the inner cover layer. A slope of the second hardness gradient is greater than a slope of the first hardness gradient.