The coupled mass transfer and chem. reactions of a gel-forming compd. in a two-phase system were studied in detail. Tetra-methyl-ortho-silicate (TMOS) is often used as a precursor in sol-gel chem. to produce silica gels in aq. systems. TMOS can also be mixed with many hydrocarbons without chem. reaction, which allows for various applications in multiphase systems. In this study, TMOS was mixed with n-hexadecane and placed together with water in small cylinders. Upon contact of the mixt. with the water, TMOS transfers completely to the aq. phase where it forms a gel through a heterogeneous reaction. NMR imaging and relaxation time measurements were employed to monitor the mass transfer of TMOS from the oleic to the aq. phase. The longitudinal relaxation time (T1) was calibrated and used to det. the concn. of TMOS in n-hexadecane during the transfer. The mass transfer rate was obtained at various temps. (25-45 DegC) and for several initial concns. of TMOS. In the aq. phase a sharp decrease in the transversal relaxation time (T2) is obsd. which is attributed to the gel reaction, in particular the formation of methanol in the initial stage. The min. in T2 indicates the gelation point, and was found to be strongly dependent on temp. and concn.