The extent of phase mixing during ageing of semi-interpenetrating polymer networks of poly(ethylene glycol) (PEG) and poly(trimethylolpropane triacrylate) (TMPTA) was determined using differential scanning calorimetry. Twelve-month-old networks containing PEG with molecular weights of 8000-20 000 g mol(-1) were more phase-separated following ageing than freshly prepared networks, while other PEG molecular weight (4000 or 100 000 g mol(-1)) systems remained stable. The networks initially formed with fast TMPTA gelation rates remained phase-stable, while those which were initially formed with progressively slower TMPTA polymerization kinetics became more phase-dispersed upon ageing.