A chemical clock (or clock reaction) is a complex mixture of reacting chemical compounds in which the onset of an observable property (discoloration or coloration) occurs after a predictable induction time due to the presence of clock species at a detectable amount. In cases where one of the reagents has a visible color, crossing a concentration threshold can lead to an abrupt color change after a reproducible time lapse.

Clock reactions may be classified into three or four types:

The simplest clock reaction featuring two reactions:

When substrate (B) is present, the clock species (C) is quickly consumed in the second reaction. Only when substrate B is all used up or depleted, species C can build up in amount causing the color to change. An example for this clock reaction is the sulfite/iodate reaction or iodine clock reaction, also known as Landolt's reaction.

Sometimes, a clock reaction involves the production of intermediate species in three consecutive reactions.

Given that Q is in excess, when substrate (P) is depleted, C builds up resulting in the change in color.

The basis of the reaction is similar to substrate-depletive clock reaction, except for the fact that rate k₂ is very slow leading to the co-existing of substrates and clock species, so there is no need for substrate to be depleted to observe the change in color. The example for this clock is pentathionate/iodate reaction.

The reactions in this category behave like a clock reaction, however they are irreproducible, unpredictable and hard to control. Examples are chlorite/thiosulfate and iodide/chlorite reactions.