Abstract This work correlates the evolution of magnetic properties with microstructure for aging of Co–Pt alloys with compositions bracketing the A1→L10+L12 eutectoid at about 60at% Pt. The magnetic properties of Co41.7Pt58.3 were observed to vary intricately with annealing time and temperature, corresponding to changes in microstructural lengthscales, phase fractions, and degree of ordering. This alloy exhibited a maximum coercivity of 4kOe and a maximum remanence ratio of 0.8, occurring at an early stage of transformation when strained L10 nanoparticles are embedded in the A1 matrix. For particle sizes below 40nm, robust exchange coupling is observed. Although the L10 nanoparticles are rigorously below the single-domain critical size for zero applied field, we argue that a form of domain wall pinning controls magnetization reversal in the peak coercivity sample. After extended aging that completely consumes the A1 phase, quantitative x-ray analysis indicates that the L12 phase is also present, suggesting that two-phase region is broader than expected. Samples with composition Co37.6Pt62.4 formed only soft ferromagnetic L12 upon aging.