The effects of temperature, pressure, and supercritical carbon dioxide (scCO(2)) concentration are investigated on the rheological behavior of poly(methyl methacrylate)/expanded graphite (PMMA/EG) composites with 1, 4, and 7 wt. % EG. The validity of the time-temperature superposition and time-pressure-scCO(2) concentration superposition principles for the PMMA/EG composites is explored. At atmospheric pressure, the time-temperature superposition principle holds for the composites with 1 and 4 wt. % EG but fails for that with 7 wt. % EG. This is demonstrated by the rheological master curves and the van Gurp-Palmen plots. The scanning electron microscopy micrographs, the Cole-Cole plots, and weighted relaxation spectra reveal that the composite with a higher content (7 wt. %) of EG exhibits a stronger interfacial interaction between the PMMA chains and EG layers and delayed chains' relaxation. The viscosity evolution with testing time for the PMMA/EG composite samples during time sweep testing demonstrates that the EG layers hinder the dissolution of scCO(2) into the PMMA matrix to some extent. Different from the time-temperature superposition principle, the time-pressure-scCO(2) concentration superposition principle is valid for all the prepared PMMA/EG composite samples. This is because the dissolved scCO(2) at the PMMA matrix-EG layer interfaces reduces the interfacial interaction between the PMMA chains and EG layers and weakens the effect of the EG layers on the chains' relaxation.