Mg–Zn–Sr alloys are suitable for biodegradable orthopaedic implants due to excellent biocompatibility and bone regeneration effect of Sr. For Sr additions exceeding 0.2 wt%, ternary phases are observed in the microstructure contrary to binary MgxZny and Mg17Sr2 intermetallics predicted by the existing phase diagrams and corrosion and mechanical properties reduce drastically. Thermal analysis employed for Mg–4Zn–xSr alloys (0 ≤ x≤2) recorded two peaks, hitherto unreported, at temperatures of ~ 514 and 409 °C with 2 wt%, and ~ 549 and 428 °C with 1 wt% of Sr added to Mg–4Zn alloy. In conjunction with SEM–EDAX and XRD analysis, one of these phases is identified as Mg11Zn4Sr3, with second one an unknown intermetallic (IM2). Formation of Mg17Sr2 phase is suppressed in the presence of Zn, and the ternary phase is favoured beyond the solid solubility of Sr in Mg. Solidification pathway of L → α-Mg + Mg11Zn4Sr3 + unknown intermetallic (IM2) has been proposed for the ternary compositions with Sr > 0.2 wt% investigated in the current study. © 2018, The Indian Institute of Metals - IIM.