Dimorphic SrSi 2 is the first compound for which the two simplest three-dimensional three-connected nets are found in its polymorphs. The cubic net of three-connected silicon atoms (SrSi 2 type of structure) can be transformed into the tetragonal one (α-ThSi 2 type of structure) by a high-pressure-high-temperature treatment. The tetragonal phase is quenchable. Heating of this phase to 600-700°C at ambient pressure results in transformation into the cubic one. At a heating rate of 20°C/min complete transformation can be achieved within 5 min in a DTA apparatus. The energy of transformation has been obtained from the peak areas of the DTA curves to -1.6 ± 0.3 kcal/mole. Although the transformation between the three-dimensional three-connected sets in SrSi 2 must be formally classified as a reconstructive one, a relatively small entropy change ( ∆S = 1 · 1 cal/deg · mole) has been calculated from the change in molar volume and p-T equilibrium conditions. Therefore, structural relations between the cubic and the tetragonal nets are discussed.