<p>The effect of the combined addition of Mo and Ti on the phase evolution of non-equiatomic (CoFeNi)_100−2.5xMo_xTi_1.5x (x = 2,4,6,8,10,12) alloys produced by high-energy ball-milling was investigated. Based on the preliminary phase stability criteria, solid solution formation is expected for each composition, a face-centred structure (FCC) phase if x is less than 10, and FCC and body-centred structure (BCC) phase in other cases. After 35&#xa0;h of milling, solid solution structures were successfully produced in all samples; an FCC, two BCC structure phases, and a small amount of Co phase were identified based on X-ray diffraction. One of the BCC phases is Mo-based (BCC1(Mo)), while the other is a Fe-based (BCC2(Fe) solid solution. Increasing the combined amount of Mo and Ti alloying up to 20 at% (x = 8), the amount of the FCC structure was dominant, while above 20 at% (x = 10), the amount of the two BCC lattice structures was predominant. The average particle sizes were smaller than 3&#xa0;μm. The specific surface area of all composition powders was less than 0.25 m²/g, which is extremely rare for a catalyst. All HEA powders containing Mo and Ti demonstrated enhanced photocatalytic activity in the decolorisation of Rhodamine B dye (RhB). The optimum conditions for RhB decolorisation were a pH of 2 and a catalyst dosage of 1&#xa0;g/L. Under these conditions, (CoFeNi)₈₅Mo₆Ti₉, (CoFeNi)₈₀Mo₈Ti₁₂, and (CoFeNi)₇₀Mo₁₂Ti₁₈ demonstrated high efficiencies of 97.6, 98.6 and 98.7%, respectively, already in the first minute of reaction.</p>