[en] The authors have applied modulation spectroscopy to study the intersubband (IS) and the interband (IB) transitions in InGaN/AlInN multi-quantum well (QW) structures with the QW width varying from 1.3 to 1.8 nm, and proposed a simple method to analyse the broadening of IS and IB transitions. With this method the measured broadenings of IS and IB transitions have been compared with theoretical calculations performed within the effective mass approximation for a QW system with various QW width fluctuations. Within the framework of this comparison it has been found that the QW width fluctuation in the investigated InGaN/AlInN QW system is close to 2 monolayers.