Mashups is an emerging technology on the Web 2.0. Users can propose favorite preferences and compose various widgets in a mashboard. The mashups selection can be performed dynamically identifying the situational set of widgets available at runtime. In this context, there are dynamic changes in user situation, defined as the configuration of accessible widgets depending on user location and schedule. The development of Situational Mashups requires mechanisms to interpret user context and to mash-up widgets. We propose a system including context representation, situation reasoning, workflow planning, and mashups optimization, called SituMash, capable of supporting the automatic composition of widgets in response to situation changes without user manual selection. Experimental results demonstrated that the system possess effective capabilities in compositiontime, ease-of-use and usefulness.