Photodynamic therapy (PDT) is an alternative to radical surgical resection for T1a or nonresectable carcinomas of the gastroesophageal junction. Besides the concentration of the photosensitizer, the light distribution in tissue is responsible for tumor destruction. For this reason, knowledge about the behavior of light in healthy and dysplastic tissue is of great interest for careful irradiation scheduling. The aim of this study is to determine the optical parameters (OP) of healthy and carcinomatous tissue of the gastroesophageal junction in vitro to provide reproducible parameters for optimal dosimetry when applying PDT. A total of 36 tissue samples [adenocarcinoma tissue (n=21), squamous cell tissue (n=15)] are obtained from patients with carcinomas of the gastroesophageal junction. The optical parameters are measured in 10-nm steps using new integrating sphere spectrometers in the PDT-relevant wavelength range of 300 to 1140 nm and evaluated by inverse Monte-Carlo simulation. Additional examinations are done in healthy tissue from the surgical safety margin. In the wavelength range of frequently applied photosensitizers at 330, 630, and 650 nm, the absorption coefficient in tumor tissue (adenocarcinoma 1.22, 0.16, and 0.15 mm-1; squamous cell carcinoma 1.48, 0.13, and 0.11 mm-1) is significantly lower than in healthy tissue (stomach 3.34, 0.26, and 0.20 mm-1; esophagus 2.47, 0.21, and 0.18 mm-1). The scattering coefficient of all tissues decreases continuously with increasing wavelength (adenocarcinoma 22.8, 12.99, and 12.52 mm-1; squamous cell carcinoma 19.44, 9.35, and 8.98 mm-1; stomach 20.55, 13.96, and 13.94 mm-1; esophagus 20.34, 12.56, and 12.22 mm-1. All tissues show an anisotropy factor between 0.80 and 0.94 over the entire spectrum. (Abstract truncated).