A new simple photoresist process for contrast enhancement of pattern profiles was published in 1987's SPIE symposium. This process, which is called REL (Resolution Enhanced Lithography), consists of conventional novolak positive photoresist process steps and a deep UV flood exposure step between image exposure and development. The effect of REL process is caused by the reduction of the solubility rate in alkaline developer in the exposed areas. In this paper, we describe the influence of the flood exposure light wavelength on profiles of photoresist and a chemical formation reaction of the insoluble layer in alkaline developer. In the experiments, three types of light sources were used which had maximum intensities at wavelengths of 254nm, 313nm, and 365nm, respectively. The resist profiles depend on the wavelength of the deep UV flood exposure light. A good resist profile with rectangular cross section was obtained with the 365nm light source. In this case, the flood exposure light reaches the internal region of the resist and the dissolution rate reduction occurs not only at the surface but also in the bulk of the resist. The chemical formation reaction of insoluble layer in alkaline developer was studied, using Differential Scanning Chromatography (DSC), Thermogravimetric analysis (TGA), Gel Permeation Chromatography (GPC), and Fourier Transform Infrared Spectroscopy (FT-IR). From the results of these analyses, it is concluded that the formation of the insoluble layer results from the esterification reaction under the deep UV flood exposure between novolak resin and unphotoreacted PAC in the exposed areas.