This research contributes to nuclear waste management by proposing a simple treatment route for high viscosity (HV) organics (oils and greases). These are currently without an industrial solution, because they are not directly pourable into a cementation matrix. Their solidification/stabilization (S/S) into geopolymers (GP) is investigated. Incorporation is achieved by assembling the HV organics (a model oil MO, an industrial oil IO or a grease G) with a low viscosity (LV) solvent, either dodecane (C12) or a mix of tributylphosphate (TBP)/C12, prior to mixing with the fresh GP. First, the minimal solvent amount is determined to achieve sufficient fluidity of the HV oil or grease mix. Two different criteria are used, one for the Newtonian oils and the other for the non-Newtonian grease. Secondly, the immobilization of 20%vol of assembled mix is performed into a GP paste or mortar. The organics bleeding, the GP setting, the rheological properties of the mix, and the mechanical performance at 28 days endogenous curing are assessed. The morphology of the oil or grease emulsion inside the hardened GP mortar is determined by 2D SEM observations and 3D quantitative X Ray micro-computed tomography. The efficiency of the immobilization is proven by cross analyzing these properties. In particular, a minimal compressive strength of 25.4 MPa +/− 4.6 is achieved for a GP mortar immobilizing 20%vol HVG assembled with TBP/C12; it corresponds to an emulsion made of 12.9%vol droplets in the 10–480 µm size range, and 7.1%vol smaller than 10 µm, without organics bleeding.