Studies of the binary mixture H2O-Ar-by means of a quasi-isochoric scanning method-have revealed the range of stability of clathrate hydrates in the high-pressure and high-temperature region. The results obtained show an extension of the decomposition curve above the melting curve of pure argon and up to the melting curve of pure ice VII. At low pressures the argon decomposition temperature first increases and then decreases with pressure, showing a local maximum and minimum temperature. At higher pressures the slope of the decomposition curve remains positive but undergoes three more breaks. Two new quadruple points have been determined; at 105 degrees C, 20 kbar (Q(4)) and at 136 degrees C, 29 kbar (Q(5)). In the lower region (below 10 kbar) the decomposition curve shows two breaks; at 31.5 degrees C, 6.2 kbar (Q(2)) and at 37.5 degrees C, 9.6 kbar (Q(3)), indicating that three different clathrate structures are formed in this region. The positions of the three-phase lines separating the two clathrate structures at lower pressure could be obtained via Raman spectroscopy since the pressure and temperature dependence of the vibrational frequencies of the coupled O-H oscillations is different for each clathrate structure.