The experimental and predicted loci extrema behavior of the isochoric heat capacity C-V was examined for water and carbon dioxide along the subcritical and supercritical isotherms and along the liquid and vapor isochores. The studies were based on a nonanalytical Helmholtz energy-volume-temperature equation (AVT, fundamental equation of state), the IAPWS-95 formulation for water, and scaling-type crossover equations of state (CREOS). The measured isochoric heat capacity data for these fluids near the critical point were analyzed to study the behavior of loci of C-V maxima and to compare these with predictions by the equations of state. A CREOS was applied to study the behavior of the isochoric heat capacity maxima in the immediate vicinity of the critical point. Good agreement with the CREOS prediction and experimental isothermal C-V maxima loci was observed near the critical point. The basic characteristic points on the C-V extrema loci curves in the P-T and rho-T planes were determined on the basis of detailed analysis of the experimental and prediction of C-V extrema loci behavior. Qualitative explanations are given for the nature of isochoric and isothermal C-V maxima-minima curves. The role of C-V extrema. loci behavior in developing high-accuracy equations of state in the supercritical region and in the study of supercritical phase-transition phenomena are discussed.