The reactions of 5-(1,2-dicarbadodecaboran-1-yl)-3-(2-pyridyl)-1,2,4-triazines with various dienophiles (2,5-norbornadiene, 1-morpholinocyclopentene, 1,2-dehydrobenzene, and 2-amino-4-phenyloxazole) were studied. It was shown that the presence of a carborane fragment in the 1,2,4-triazine ring causes an atypical reaction with dienophiles without the formation of the expected aza-Diels–Alder reaction products except for the reaction with 2,5-norbornadiene. The reaction of 5-(1,2-dicarbadodecaboran-1-yl)-3-(2-pyridyl)-1,2,4-triazines with 2-amino-4-phenyloxazole unexpectedly led to the formation of the corresponding 4,5-dihydro-1,2,4-triazines. One of the previously described procedures for the direct introduction of a carborane residue into the C5 position of 1,2,4-triazines was optimized.