Two in-situ test techniques were used to obtain insight into the isothermal precipitation kinetics of c¢¢ and c¢ in superalloy 718. The first method consisted of measurements of Young's modulus using a dynamic-resonance method (DRM), and the other comprised determination of the evolution of the lattice parameter of the c-matrix phase via neutron diffraction. For both techniques, solution-treated-and-water-quenched samples were heated to a nominal test temperature of 923 K, 953 K, 1013 K, or 1053 K and held for a time of~5 to 50 hours, at the end of which a near-steady condition in terms of an apparently-constant modulus/lattice parameter had been achieved. The observations from each test technique suggested sequential periods of rapid initial increase in volume fraction followed by a gradually-decreasing rate of change. The data were converted to transformed fraction as a function of time and interpreted in terms of phenomenological (JMAK) and mechanistic (nucleation-and-growth) models. From the former approach, Avrami exponents which decreased from approximately unity at the lower two temperatures to~0.5 at the highest temperature, were deduced. The transformed fraction-versus-time behaviors were well replicated using fast-acting numerical simulations based on the mechanistic model. These simulations highlighted the competition between nucleation and growth in determining overall transformation kinetics.