The conversion of sugars to 5-hydroxymethylfurfural (HMF) over solid acids in water represents an environmentally and separation-friendly route to an important platform molecule. In particular, the conversion of sucrose attracts increasing attention because it is cheaper and more widely available than glucose and fructose. Sucrose can undergo rapid hydrolysis to the two monosaccharides, however conversion mechanisms and interactions with solid acids remain unclear. Here, it is shown that niobium oxides possess Bronsted acid (BA) and Lewis acid (LA) sites of tunable quantity and strength, dependent on their structure and morphology. By systematically studying these acid catalysts, it is revealed for the first time that both acid type and strength are significant for the sugar conversion: Fructose reaction is catalyzed by BA, with weaker BA sites being more selective toward HMF. Glucose conversion to HMF involves an additional isomerization step to fructose, which can be catalyzed by both LA and strong BA but LA is more efficient. Sucrose is shown to be easily hydrolyzed into glucose and fructose under the reaction conditions and HMF is formed from the further conversion of the two sugars. It is demonstrated that mesoporous niobium oxide gives the highest HMF yield for sucrose conversion among all niobium oxides due to balanced BA and LA sites with appropriate acid strengths. (C) 2016 Elsevier Inc. All rights reserved.