The shear-thickening behavior of reversibly cross-linked guar network is studied using rheological and particle imaging velocity measurements. New evidence suggests that both shear-induced increase in crosslink density and non-Gaussian chain stretching are possible mechanisms for shear thickening. Which mechanism plays a predominant role depends on the applied shear rate (gamma) over dot(a) and shear time. At (gamma) over dot(a) not too much larger than 1/tau, where tau is the network relaxation time, shear thickening is mainly caused by the increase in crosslink density. At higher shear rates, shear thickening is initiated by the increase in chain density at short times, and non-Gaussian chain stretching occurs at longer times. It is demonstrated that the linear elastic modulus measured for the shear-thickening state and its relaxation time can be used to discriminate between non-Gaussian chain stretching and shear-induced crosslinking mechanisms. The detection of a linear step strain regime where the measured modulus does not change with the strain amplitude indicates the absence of non-Gaussian chain stretch. When chains are stretched into the non-Gaussian regime, the relaxation time becomes smaller whereas relaxation time remains unchanged if only crosslink density increases. At high shear rates, flow may become unstable with bulk fracture, shear banding, and continuous flow occurring randomly as revealed by the velocity profile across the flow cell gap. (C) 2014 The Society of Rheology.