Not D76h which does not contain any Hydroquinone. There are several explanations around about what happens with D76 as it ages. To me the most likely story seems to start from the known fact that Hydroquinone kneeds to be oxidised to Semiquinione to act superadditive on Metol (i.e. as a regenerator, Junge & Hübner, Photographische Chemie ist a good reference on that). With aged, slightly oxidised D76 there will be more Semiquinone in the solution from the start. This fits nicely to the observation that the rise in contrast with aging D76 is continuous and slow.
Originally Posted by geraldatwork
In Anchel's and Troop's film developing cookbook it is said that the pH of the solution will rise with age untill it reaches the level were Hydroquinone starts to reduce silverhalide by itself. Having a had a lesson or two in chemistry I'd expect a marked and steep increase at around that pH and a much slower change above and below that value but this is not how it looks.