Linear viscoelastic properties of carbon black (CB) suspensions in a mixture of a rosin-modified phenol resin-type varnish (Varnish-1)/an alkyd resin-type varnish (Varnish-2), which exhibited a sol-gel transition on an increase in CB concentration, were investigated from 30A degrees C to 80A degrees C. The viscoelastic properties were reversible from 30A degrees C to 60A degrees C. In contrast, at temperatures above 60A degrees C, the storage (G') and loss (GaEuro(3)) moduli were irreversible and increased significantly with increasing temperature. This increase in the moduli is due to a change of the dispersion state to agglomerated state by heating. The agglomerated state was held, when the suspensions were lowered at 30A degrees C. However, the G' and GaEuro(3) recovered to the original values upon shearing. This heat-induced gelation should be a universal feature for suspensions of weakly attractive particles. The temperature and shearing histories of the suspensions were discussed in relation to adsorption of polymeric component in the varnish on the CB particles.