A combination of calculations using the FactSage software and measurements using a number of experimental techniques was explored to assess the solidification characteristics of ternary Mg-Zn-Nd alloys along with the commercial grade ZEK100 and determine the role of rare earth metal neodymium. For each chemical composition tested, the solidification under equilibrium and non-equilibrium conditions affected the type and volume fractions of phases formed. Thermal analysis identified two major reactions during solidification: formation of α-Mg dendrites followed by the eutectic transformation. For a constant Zn content of 2% and 4%, an increase in Nd content in the range of 1–2% caused a reduction in liquidus temperature but increase in solidus and eutectic temperatures. As verified by controlled solidification experiments the cooling rate during solidification affected the refinement of alloy microstructure, a volume fraction of intermetallic precipitates and their distribution. There was no obvious influence of Nd content on the value of secondary dendrite arm spacing for all cooling rates examined. However, there was an influence of Zn where an increase of its content from 1% to 4% halved the average secondary dendrite arm spacing. The beneficial role of Zr is confirmed and a presence of 0.25%Zr in ZEK100 caused a dendrite refinement comparable to that achieved through an increase in a cooling rate from 30 °C/s to 110 °C/s. The role of small-additions of Nd and Zn in design of new magnesium alloys, specifically optimized for twin roll casting, is discussed. •Some discrepancies in phases predicted by FACTsage software and identified in diluted Mg-Zn-Nd castings were revealed.•There was no obvious influence of Nd content on the secondary dendrite arm spacing for all cooling rates examined.•An increase of Zn content from 1% to 4% reduced almost twice the average secondary dendrite arm spacing.•0.25%Zr caused dendrite refinement comparable to that achieved by increase in cooling rate from 30 to 110 °C/s.