Aim: The aim of this study was to assess the shaping ability and surface topography of WaveOne Gold (WOG) reciprocating and OneShape (OS) rotating single file systems. Materials and methods: Thirty mesiobuccal canals with severe curvature (25 to 40 degrees) were assigned to two equal groups according to the instrument used for preparation - WOG or OS files. The shaping ability was assessed by measuring canal transportation and centring ratio. Data were statistically analysed using an independent sample t test to compare between more than two groups in independent samples. A paired sample t test was used to compare between two groups in dependent samples. Surface topography of new and used instruments was analysed using scanning electron microscopy (SEM) and the chemical profile of those instruments was inspected using energy dispersive X-ray spectroscopy (EDS). Results: The differences between both instruments were not of statistical significance at any level for canal transportation and centring ratio. However, when OneShape was used, the mean centring value at 3 mm (0.62 ± 0.24) was significantly higher than at 5 mm (0.35 ± 0.25) (P = 0.02). SEM examination revealed machining grooves in all new instruments. After instrumentation, OS showed unwinding of the flutes with deformation at the edges; while WOG showed very little blunting at the edges and tip. EDS analysis showed that the as-received files were composed mainly of nickel and titanium for WaveOne Gold and OneShape; with traces of iron and nitrogen. However, oxygen constituted 6.58 wt% of the WOG composition. After usage, the elementary composition of WOG remained almost the same. Conversely, OneShape instruments had less nickel and titanium, and more nitrogen. Moreover, oxygen appeared as a new element in the composition Conclusions: Both instruments are considered safe for instrumentation of severely curved root canals. However, surface characteristics and chemical profile suggest that it is safer to use OneShape file for only a single molar with severely curved canals. Keywords: EDS, OneShape, reciprocation, SEM, severe canal curvature, shaping ability, WaveOne Gold