We studied the mechanism of formation of cation-nonstoichiometric compositions of the Tm2-xMnxO3 (x = 0.96, 0.98, 1, 1.02, 1.04, 1.06) solid solution with a hexagonal crystal structure. The entire region of homogeneity of compositions with an excess/deficiency of Mn and Tm was covered. Their structural and magnetic properties were studied by X-ray diffraction and magnetometry. The analysis of the concentration dependence of the structural parameters allows us to conclude that the formation of compositions with an excess of Tm occurs through the formation of vacancies in the Mn sublattice. The mechanism of formation of solid solutions with an excess of Mn includes the reaction of manganese disproportionation: 2Mn3+=Mn4++Mn2+. The Mn2+ ions formed as a result of this reaction occupy thulium sites, while manganese sites are occupied by Mn3+ and Mn4+ ions. Magnetic properties have been studied as functions of temperature in the temperature range 5 – 300 K. It was established that all the samples undergo a transition from the high-temperature PM state to AF one at T = TN1. The temperature TN1 both at x < 1 and x > 1 is reduced and takes a value in the range of 81–84 K. With decreasing temperature (T < TN1) in samples with x = 0.96, 1.04, 1.06 an additional first-order transition to a new AF state is observed in the vicinity of T∼45 K. The Curie-Weiss temperature ΘCW monotonically decreases for compositions with x < 1. At the same time with x > 1 a nonlinear dependence of ΘCW (x) is observed.