This study shows that the molecular chaperone BiP regulates the ER stress sensor Ire1 by binding directly to its oligomers: when Ire1's luminal domain engages unfolded proteins it oligomerises and exposes BiP-binding motifs, and BiP then binds these substrate-bound oligomers in an ATP-dependent way to help deactivate Ire1 once stress resolves. Flow Induced Dispersion Analysis (FIDA), on a Fida 1 instrument, was used to measure the apparent hydrodynamic radius of the Ire1 luminal domain, giving a solution readout of how substrate and BiP shift its oligomerisation, complementing NMR, microscopy and calorimetry. It is an example of using FIDA to measure protein oligomerisation and size in solution.