Lab and pilot scale batch cultivations of a CHO K1/dhfr(-) host cell line were conducted to evaluate on-line multifrequency permittivity measurements as a process monitoring tool. The beta-dispersion parameters such as the characteristic frequency (f(C)) and the permittivity increment (Delta epsilon(max)) were calculated on-line from the permittivity spectra. The dual-frequency permittivity, signal correlated well with the off-line measured biovolume and the viable cell density. A significant drop it? permittivity was monitored at the transition from exponential growth to a phase with reduced growth rate. Although not reflected in off-line biovolume measurements, this decrease coincided with a drop in OUR and was probably caused by the depletion of glutamine and a metabolic shift occurring at the same time. Sudden changes in cell density, cell size, viability, capacitance per membrane area (C-M), and effects caused by medium conductivity (sigma(m)) could be excluded as reasons for the decrease in permittivity. After analysis of the process data, a drop in f(C) as a result of a fall in intracellular conductivity (sigma(i)) was identified as responsible for the observed changes in the dual frequency permittivity signal. It is hypothesized that the beta-dispersion parameter f(C) is indicative of changes in nutrient availability that have an impact on intracellular conductivity sigma(i). On-line permittivity measurements consequently not only reflect the biovolume but also the physiological state of mammalian cell cultures. These findings should pave the way for a better understanding of the intracellular state of cells and render permittivity measurements an important tool in process development and control. (C) 2009 American Institute of Chemical Engineers Biotechnol. Prog., 26: 272-283, 2010