The new timescale has a physical meaning: t * s2 is roughly characteristic of the time it takes s 2 to reach s * 2 . Three questions must now be addressed: can the new timescale be used as a matching timescale? Does the introduction of this new timescale provide additional insight into the inner and outer solutions? How do the equations scale with respect to t * s2 , and does the rescaling warrant an additional criteria for the validity of the QSSA? First, let us rescale just the mass action equations of the indicator reaction with respect to T * = t/t * s2 :