A one-step, gas-phase, catalyst-free detonation of hydrocarbon (C2H2) method was developed to produce gram quantities of pristine graphene nanosheets (GNs). The detonation of C2H2 was carried out in the presence of O2. The molar ratios of O2/C2H2 were 0.4, 0.5, 0.6, 0.7 and 0.8. The obtained GNs were analyzed by XRD, TEM, XPS and Raman spectroscopy. The GNs are crystalline with a (002) peak centered at 26.05° (d = 0.341 nm). TEM shows that the GNs are stacked in two to three layers and sometimes single layers. An increase in the size of the GNs (35-250 nm) along with a reduction in defects (Raman I(D)/I(G) ~ 1.33-0.28) and specific surface area (187-23 m(2) g(-1)) was found with increasing O2 content. The high temperature of the detonation, ca. 4000 K, is proposed as the cause of graphene production rather than normal soot. The method allows for the control of the number of layers, shape and size of the graphene nanosheets. The process can be scaled up for industrial production.