A technique for the in-process measurement of polymer stationary phase growth inside fused silica capillaries during the fabrication of monolithic porous layer open tubular (monoPLOT) columns is presented. In this work, capacitively coupled contactless conductivity detection (C4D) was applied as an online measurement tool for porous polymer layer growth within fused silica capillaries. The relationship between effective capillary diameter and C4D response was investigated for two polymers, butyl methacrylate–ethylene dimethacrylate (BuMA–EDMA) and polystyrene–divinylbenzene (PS–DVB) over a range of capillary diameters and layer thicknesses. The described technique can be used with both thermal and photo-initiated approaches for monoPLOT fabrication and provides an accurate, realtime measurement of the porous layer growth within the capillary, which should vastly improve columnto-column reproducibility. The technique was shown to be very precise, with a measured %RSD < 10%.