Background: Bioelectrical impedance analysis (BIA) is a validated method to assess body composition in persons with fluid homeostasis and reliable body weight. This is not the case during critical illness. The raw BIA markers resistance, reactance, phase angle, and vector length are body weight independent. Phase angle reflects cellular health and has prognostic significance. We aimed to assess the course of phase angle and vector length during intensive care unit (ICU) admission, and determine the relation between their changes (Δ) and changes in body hydration. Methods: A prospective, dual-center observational study of adult ICU patients was conducted. Univariate and multivariable regression analyses were performed, including reactance as a marker of cellular mass and integrity and total body water according to the Biasioli equation (TBWBiasioli) and fluid balance as body weight independent markers of hydration. Results: One hundred and fifty-six ICU patients (mean ± SD age 62.5 ± 14.5 years, 67% male) were included. Between days 1 and 3, there was a significant decrease in reactance/m (−2.6 ± 6.0 Ω), phase angle (−0.4 ± 1.1°), and vector length (−12.2 ± 44.3 Ω/m). Markers of hydration significantly increased. Δphase angle and Δvector length were both positively related to Δreactance/m (r2 = 0.55, p < 0.01; r2 = 0.38, p < 0.01). Adding ΔTBWBiasioli as explaining factor strongly improved the association between Δphase angle and Δreactance/m (r2 = 0.73, p < 0.01), and Δvector length and Δreactance/m (r2 = 0.77, p < 0.01). Conclusions: Our results show that during critical illness, changes in phase angle and vector length partially reflect changes in hydration.