Second-order phase transformation in double complex salt (DCS) [Pd(NH3)4][RuNO(NO2)4OH] was examined by DSC, single-crystal diffraction and powder diffraction. All data confirm the reversible “order-disorder” phase transition without an experimentally detected sharp volume change in the temperature range of 343-363 K. At low temperature, the less symmetric triclinic phase (P1¯) is stable, while the monoclinic phase (P2/m) is stable at high temperature. The main difference between crystal structures is the disorder of the ruthenium anion in the monoclinic phase due to the presence of the ruthenium atom in a special position. The additional symmetry plane in theP2/mphase includes ON-Ru-O atoms, resulting in the rotational disorder of NO2groups. The entropy of transformation determined by differential scanning calorimetry corresponds to order-disorder transformation with a 3-fold increase in microstate number.