Grid computing is diverse and heterogeneous in nature, spanning across multiple domains where resources are not owned or managed by a single administration. This brings about many challenges to Grid resource management and exposes the user to the Grid middleware complexities. Thus this research develops a user-centric resource broker that insulates the users from the Grid complexities, alleviating them from the burden of having to know the various mechanisms of the Grid middleware. The broker is based on the SNAP (Service Negotiation and Acquisition Protocol) framework and focuses on applications that require resources on demand. It is important for applications that require resources on demand to reserve the necessary resources within the minimum time possible. Thus the work in this thesis has developed a three-phase commit protocol which enhances the traditional twophase commit protocol. Performance evaluation has been carried out to evaluate the SNAP-based resource broker using the traditional two-phase commit protocol and the newly developed three-phase commit protocol. The evaluation has been conducted on a local Grid test-bed, a distributed Grid infrastructure (the White Rose Grid) and through mathematical modelling and simulation. Throughout the evaluation, the SNAP-based resource broker using the three-phase commit protocol provides a significant performance enhancement, over the use of the traditional twophase commit protocol, in terms of the time taken between submission (to the broker) of user requirements and the job beginning execution.