Solvothermal techniques are widely used to exfoliate many two-dimensional materials, but the formation mechanisms of these nanomaterials have not been clearly revealed. Herein, we discovered the dissociation of hexagonal boron nitride (h-BN) in solvothermal exfoliation evidenced by the formation of B(OH)3, NH4B5O8·4H2O, and (NH4)2B10O16·8H2O. In the selected solvents, the lateral sizes of the formed boron nitride nanosheets (BNNSs) are increased in the order of N-methyl-2-pyrrolidone (NMP), N,N-dimethylformamide (DMF), acetonitrile (MeCN), and isopropanol (IPA), suggesting the decreased dissolving abilities of these solvents to h-BN in turn. The dissociation behaviors are the properties of solvents themselves, but the inclusion of lithium chloride (LiCl) and cetyltrimethylammonium bromide (CTAB) can elevate the dissociation degree and yield BNNSs with smaller lateral sizes due to the intercalating effects. The cation-π interactions make CTAB more effective in obtaining uniform BNNSs than using the neutral halogenated hydrocarbons as assistant reagents. The dissociation abilities of the solvents have strong relationships with the surface tension, Hansen solubility parameters distances (Ra), and polarities, whereas there is little relevance with the pressures. Meanwhile, we also observed the cracking of CTAB and the polymerization of MeCN in these reactions. Our findings indicate that the impurities are prone to be attached to the BNNSs exfoliated by the solvothermal route.