When the reaction is exothermic (negative ΔH) but undergoes a decrease in entropy (negative ΔS), it is the enthalpy term that favors the reaction. In this case, a spontaneous reaction is dependent upon the TΔS term being small relative to the ΔH term, so that ΔG is negative. Entropy is the amount of disorder in a system. Negative entropy means that something is becoming less disordered. In order for something to become less disordered, energy must be used. This will not occur spontaneously. If ΔH and ΔS are both negative, ΔG will only be negative below a certain threshold temperature and we say that the reaction is only spontaneous at 'low temperatures.