Many macromolecules in the cell function by forming multi-component assemblies. We have applied the technique of small angle neutron scattering to study a nucleic acid-protein complex and a multi-protein complex. The results illustrate the versatility and applicability of the method to study macromolecular assemblies. The neutron scattering experiments, complementing X-ray solution scattering data, reveal that the conserved catalytic domain of RNase E, an essential ribonuclease in Escherichia coli (E. coli), undergoes a marked conformational change upon binding a 5'monophosphate-RNA substrate analogue. This provides the first evidence in support of an allosteric mechanism that brings about RNA substrate cleavage. Neutron contrast variation of the multi-protein TIM10 complex, a mitochondrial chaperone assembly comprising the subunits Tim9 and Tim10, has been used to determine a low-resolution shape reconstruction of the complex, highlighting the integral subunit organization. It shows characteristic features involving protrusions that could be assigned to the six subunits forming the complex.