Most existing techniques for regulating the ink flow in offset presses rely oil density measurements carried out oil specially printed patches. In the present contribution. we develop a methodology to deduce ink thickness variations from spectral measurements of multichromatic halftone patches located within the printed page. For this purpose, we extend the Clapper-Yule spectral reflectance prediction model by expressing the transmittance of the colorants composed of superposed inks as a function of the ink transmittances and of fitted ink layer thicknesses. We associate to each ink all ink thickness variation factor. At print time, this ink thickness variation factor can be fitted to minimize a difference metric between predicted reflection spectrum and measured reflection spectrum. The ink thickness variations deduced from multichromatic halftones allow to clearly distinguish between normal ink volume, reduced ink volume, or increased ink volume. This information can then he used for performing control operations on the printing press. (C) 2009 Wiley periodicals, Inc. Col Res Appl, 34, 432-442. 2009; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/col.20541