The　high-temperature　oxidation　behaviours　of　AlxHfMoNbTi-series　refractory　high-entropy　alloys　with　an　Al　content　ranging　from　0　to　20　at.%　(x=0-1)　were　tested　at　700-1000　degrees　C.　An　in　situ　oxidation　experiment　was　performed　at　25-900　degrees　C　using　environmental　transmission　electron　microscopy　(ETEM)　to　reveal　the　oxidation　mechanism.　The　addition　of　Al　to　the　HfMoNbTi　alloy　effectively　increased　the　alloy　strength　and　hardness.　The　improvement　in　oxidation　resistance　by　Al　addition　is　attributed　to　the　formation　of　mixed　oxide　structures　instead　of　layered　oxides.　Mo-rich　precipitates　are　preferentially　oxidized,　and　their　volatilization　at　the　metal/　oxide　interface　accelerates　alloy　oxidation.