In this work, the evolution of microstructure and fundamental mechanical properties with Mo concentration in the arc-melted (TiZrNbTa)100-xMox (0 ≤ x ≤ 20) high-entropy alloys (HEAs) are investigated. The arc-melted (TiZrNbTa)100-xMox alloys structurally consist of two bcc phases. The change in volume fractions of two phases in the microstructure is insignificant with Mo concentration, at levels of ∼75% for bcc1 and ∼25% for bcc2. The increases in microhardness and Young's modulus of the alloys linearly scale with Mo concentration. To compromise the strength and ductility, the (TiZrNbTa)90Mo10 exhibits an optimal combination of stiffness (E = 137 GPa), compressive yield strength (σy = 1370 MPa) and deformability (εp ≈ 25%). In addition, it is indicated that dislocation widths in bcc lattice of refractory HEAs are insensitive to the alloying complexity, reflecting that the Peierls barrier is excluded as a predominant factor of strengthening. Furthermore, a simple model is proposed to reveal the solid solution hardening (SSH) in bcc refractory HEAs, in which the solid solutions are treated as an imposition of distortion-free and distorted lattices. By applying this model to archived refractory HEAs, the predicted yield strength agrees well with experimentally measured values. It provides a simple empirical tool used for predicting the strength of bcc refractory HEAs and to assist new alloy design. •Arc-melted (TiZrNbTa)Mo high-entropy alloys (HEAs) consist of two bcc phases in microstructure.•Microhardness and Young's modulus of the alloys increases with Mo concentration.•(TiZrNbTa)90Mo10 exhibits an optimal combination of compressive strength and deformability.•A simple model is proposed to reveal the solid solution hardening in bcc refractory HEAs.•Yield strength predicted with this model is good in agreement with experimental data.