As with acids, there are two types of base catalysis; specific and general.

Specific Base Catalysis

With specific base catalysis, the rate is again found to be directly related to the pH.  The only species capable of catalysing the reaction is OH.  An example of a base catalysed reaction:

This is the reverse of an aldol condensation.

Again, as with the specific acid catalysis, there is found to be rapid, reversible removal of a proton before the RDS.

General Base Catalysis

In general base catalysis (as with the analagous acid catalysis), bases other than OH are potential catalysts. e.g:

As we might expect by analogy with general acid catalysis, the removal of the proton occurs with the RDS.