Rheological measurements are performed on highly concentrated alumina gels. By using an in situ mechanism based on enzyme-catalyzed internal reactions, we are able to form gels of highly concentrated particles without disturbing the microstructures that develop during the gelation process. These gels are produced by two different destabilization mechanisms: Either the pH of the suspension is shifted toward the isoelectric point (ΔpH method) or the ionic strength of the suspension is increased at a constant pH (ΔI method). The two destabilization mechanisms lead to gels of significantly different microstructures. We find notable differences in the rheological behavior of the two systems, suggesting a bond-bending mechanism for stress transmission in the case of ΔpH gels and a bond-stretching mechanism in the case of ΔI gels. In addition, for both kinds of gels we compare the in situ properties to those obtained after altering the microstructure by shearing. Results suggest an increase in elastic and yield properties of concentrated particle gels with decreasing homogeneity of their microstructures. © 2004 American Institute of Chemical Engineers AIChE J, 51: 134–141, 2005