This Nature Communications study shows that capillary-flow measurements can rapidly and quantitatively characterize protein liquid–liquid phase separation (LLPS). The authors introduce Capflex, a method built on Flow Induced Dispersion Analysis (FIDA) and run on the Fida 1 instrument, which injects a sample into a temperature-controlled capillary where LLPS occurs. As droplets pass the detector they produce fluorescence spikes, while the baseline reports the dilute-phase protein concentration. From this, Capflex determines dilute-phase concentrations, relative droplet size distributions, droplet formation and maturation kinetics, and the binding affinity between the phase-separating protein and modulating compounds. The team applied it to Ddx4n1, an ssDNA/RP3 coacervate system, and α-synuclein, quantifying the liquid-to-solid transition into amyloid fibrils. The approach offers high information content, throughput and versatility for studying biomolecular condensates.