Anodisation of n-InP in KOH results in the formation of porous layers with a finite thickness. We propose the reason for the cessation of porous etching is the formation of insoluble precipitates within the pores. Electron micrographs of mature porous layers show significant precipitates within the porous structure. An in-situ microscopy study of the surface of InP electrode during anodisation reveals the formation of a layer on the surface. This layer emerges from a point on the surface and quickly spreads across it. A likely source of this layer is the spreading of precipitation from the etch-products saturated solution within the porous layer. However, as we explain, once a complete porous layer has formed, there should be no significant increase in mass transport requirements through the porous network, leaving the exact mechanism of the precipitation unclear.