The use of base isolation to help mitigate and reduce the effects of earthquake excitations has become common place on many important structures. There is also a larger amount of heavier machinery and equipment being stored in some of these important structures; this means that there is a possibility that there are mass irregularities with in a structure. While the response of structures that have been base isolated has been studied they are typically design with floors having a uniform mass. This thesis investigates how mass irregularities affect the response of the floor accelerations and interstorey drifts within a flexural structure with and without a base isolation unit. The ductility demand of the isolator unit is also investigated at during the course of the analysis. The reason for observing the response of the structure is because often in building design there is a need to have floors that have larger masses then the rest of the structure, and understanding how these mass irregularities affect the response of the structure, then the designing of such structures will be simpler during the initial concept stage.