<p>Previous research into refractory high-entropy alloys (RHEAs) often focused on optimizing alloys with solid solution phases by adjusting elemental compositions and refining microstructure. To be suitable for critical structural applications, formation of secondary phases, such as those seen in the microstructures of many superalloys, is an area which is still in the early stages of exploration for RHEAs. In this work, a new Mo₅Nb₃₅Ti₃₀V₃₀ RHEA is manufactured <i>via</i> laser powder bed fusion and subsequently heat treated, inducing the formation of a TiCN phase, initially on cell and grain boundaries (GBs) after 1 hour. After prolonged 24-hour heat treatment the TiCN on the GBs coarsens and the cellular substructure is removed. Samples are then compression tested, all showing ductile failure. Due to the strengthening caused by interstitial elements in the body-centered cubic (BCC) matrix phase, recovery of the cellular substructures and micron-scale TiCN on GBs, the 24-hour heat-treated samples showed increased compressive strength and ductility compared to the as-built samples. TiCN largely grows at a 45 deg misorientation angle about the [100] axis in the BCC matrix phase, hence Kernel average misorientation (KAM) maps show dislocation pile up at the phase boundaries and at the high angle grain boundaries in the recovered microstructure. Susceptibility of RHEAs to atmospheric interstitial infiltration is a concern in the RHEA field; however, this work shows that, if controlled, exposure to these elements can result in beneficial dual-phase microstructures, interstitial strengthening and improved material properties as a result.</p>