By identifying and characterising the six phases present in the GSF/PEG melt crystallisation product, the first case of polymorphism within ICs built from the same clinical drug-polymer combination could be observed. The discovery of the new IC demonstrates that drug molecules that form more than one channel solvate can form more than one IC with the same linear polymer, and these ICs may exhibit large differences in stability. The stability of ICs has previously been altered by changing the type or molecular weight of the guest polymer.