I encountered a problem with a simple subs in Maple 15.01:

Q:=1/(a*l);

subs(a*l=t,Q);

This fails to perform the substitution and gives 1/(a*l) instead of 1/t.

However, it seems to work in certain form, for example:

Q:=exp(a*l);

subs(a*l=t,Q);

produces exp(t) correctly.

Furthermore,

Q:=1/(a*l)*exp(a*l);

subs(a*l=t,Q);

gives exp(t)/(a*l). Strangely, the substitution in the exponential is correct...