Secondary-active trans-membrane transporters facilitate the movement of essential substrates, such as neurotransmitters or metabolites, across the cell membrane. Disruption of these vital processes is implicated in severe diseases in humans, e.g. Parkinson’s disease. Our aim is to investigate the structural basis of the transport mechanism of secondary-active transporters, using the bacterial sodium-hydantoin transporter Mhp1 [1] as a model system examined by serial time-resolved crystallographic methods. Mhp1 has been previously crystallized in two crystal forms in its outward (P2₁2₁2₁) and inward (P6₁) facing conformations, and soaking experiments have so far only shown substrate bound in the outward facing cavity, suggesting that crystal packing may limit the ability of the transporter to complete its full structural cycle in the crystalline state. Interestingly, SEC-SAXS experiments suggest that different buffer conditions result in the formation of differently arranged small oligomers (< 6 monomers) that may result in distinct crystal forms. We are therefore exploring the use of SEC-SAXS as a tool to guide the choice of crystallization conditions in order to obtain crystal packing arrangements that will enable the structural transitions associated with substrate transport.