Large volcano sector and flank collapse events are amongst the most devastating geological processes and thus pose an important threat to nearby and far away populations. These landslides can incorporate up to hundreds of cubic kilometres and travel distances of over 100 kilometres. Such events have happen on average every 25 years over the last half millennia. Thirteen large volcanic landslides have been identified in a total of eight volcanic edifices in Costa Rica. Most previous studies in this field have focused on the dynamics of emplacement of volcanic landslide avalanches and few have looked into the mechanism triggering the failures. Many such mechanisms have been proposed including volcanic phenomena such as: magmatic intrusions, hydrothermal alteration, caldera collapses or transient stress from seismicity; and non-volcanic phenomena such as: geologic, morphologic, hydrogeologic, geotechnical and tectonic factors. However, in order to quantify the effect of each potential destabilising process requires knowledge of the mechanical properties of the materials comprised within the slopes. A yet unresolved issue in this matter is the geotechnical classification of volcanic materials. Herein we propose a new classification of materials into four units – lavas, autoclastic breccias, pyroclastic rocks and volcanic soils – further subdivided on the basis of hydrothermal alteration, welding and interlocking. A combination of new data and normalised reworked data from the literature are presented for each geotechnical unit. Material strengths from the geotechnical characterisation have then been incorporated into slope stability analyses. Both finite element (FEM) and limit equilibrium models (LEM) have been used in a detailed parameter sensitivity analysis of the destabilising effect of volcanic and non-volcanic process.