Solid-state phase transformations in the γ-loop of the binary Fe-P system were studied using differential scanning calorimetry (DSC) and high-temperature laser scanning confocal microscopy (HT-LSCM). In total, eight alloys with varying P content from 0.026 to 0.48 mass pct. P were investigated in the temperature range of 800 °C to 1450 °C. The first part of the present work deals with the critical evaluation of the approach to couple DSC experiments and HT-LSCM observations in order to characterize bcc/fcc phase equilibria in Fe-based γ-loops. The phase transformation temperatures of a selected alloy with 0.394%P were analyzed by DSC and HT-LSCM and compared with results of the well-established techniques of dilatometry and high-temperature X-ray diffraction (HT-XRD). Then, the overall phase boundaries of the γ-loop were reconstructed by HT-LSCM and DSC data and the phase diagram was compared with thermodynamic assessments from literature. Finally, the quantitative phase fractions of fcc and bcc at 0.394%P were analyzed by Rietveld refinement at temperatures of 1050 °C, 1100 °C and 1150 °C using in-situ HT-XRD. Although the phase boundaries of the γ-loop and phase transformation temperatures have been reproduced accurately by recently published thermodynamic optimizations, larger deviations between HT-XRD measurements and the calculations were identified for the phase fraction prediction. The present work clearly demonstrates that coupling DSC and HT-LSCM is a powerful tool to characterize γ-loops in steel for future research work.