It is known that viscosity represents one of the most important properties for a quantitative interpretation of the processes that convert a glass into a glass-ceramic, giving information about the possible physicochemical behaviour of the material during heat treatment, so that the optimum heat treatment cycles can be determined. Many data are available on the viscosity and heat capacity of the MgO-CaO-A1203-SiO2 system, which forms diopside and anorthite glasses, since these components, being some of the major components in magmas, have been of great interest for the understanding of igneous petrological processes [1-4]. Being particularly interested in the glass-ceramic field, in a previous paper we investigated the natural trend towards nucleation of three compositions belonging to the region in the glass formation in the quaternary MgO-CaO-A1203-SiO2 system where anorthite and diopside in 3/1, 1/1 and 1/3 ratios are stable [5]. In this letter we report an investigation on the flow behaviour of the previously investigated anorthite-diopside glasses over a temperature range (700-900 °C) higher than the glass transition temperature (Tg) with two different techniques: beambending and fibre elongation in isothermal conditions, in order to determine the activation energies for viscous flow to compare with that for crystallization, since viscous flow is the most important mass transportation phenomenon in glass devetrification. The time-temperature-transformation ( T T T ) curves for the different glasses are also presented (and discussed in relation to the viscosity data). The results are compared with lowwiscosity data obtained for systems with similar compositions [1, 4] and with those from other researchers on the same compositions using different viscometers [6]. Two different compositions in the system MgO-CaO-A1203-SiO 2 with the molar fractions 0.25CaO-0.20MgO-0.05A1203-0.50SiO2 and 0.25CaO-0.14MgO-0.11A1203-0.50SiO2 (hereafter abbreviated as A and B, respectively) and considered in a previous work [5], separate glass-ceramics containing calcium(II) feldspar (anorthite) and a pyroxene (diopside) in 1/3 and 1/1 ratios, respectively. The specimens were prepared from reagent grade powders of silica, alumina, magnesium carbonate and calcite. The samples were melted in alumina crucibles at 1450 °C for 1 h. Fibres for the elongation viscosimeter were drawn from the hot melt and not annealed, whereas samples for the beam bending were obtained in graphite moulds