The impact of roughness on nonlinear mixed convective nanofluid flow past a sphere is analysed in the presence of nonlinear density variations. This study is found to be innovative as it investigates the effects of nanoparticles, nonlinearity and surface roughness on mixed convective flow past a sphere with three diffusive components. The problem is modelled in the form of nonlinear partial differential equations that are dimensional in nature. This set of equations is transformed to dimensionless form by applying non-similar transformations. The technique of Quasilinearization is employed to linearize the transformed set of equations and then the implicit finite difference scheme is used for further simulation to get the required numerical solutions. The graphical presentation of numerical results exhibit that the friction, heat, mass and nanoparticles mass transfer rates at the surface of sphere increase along with the fluid's velocity due to the roughness of the surface, while the fluid's temperature reduces, significantly. The steep jump in the fluid's velocity near the wall is observed due to the surface roughness. The present analysis reveals that separation of boundary layer can be delayed with the proper selection of roughness and mixed convection parameters. Also, the third diffusing component, namely, liquid oxygen influences the fluid flow significantly. That is, the introduction of liquid oxygen diffusion into the liquid hydrogen diffusion diminishes the species concentration boundary layer, while it increases the corresponding mass transfer rate. (C) 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.