The absolute flux calibration of three of the four ISO instruments (CAM, PHT - at short wavelengths - and SWS) is based primarily on results from the Ground-Based Preparatory Programme (GBPP, Jourdain de Muizon & Habing 1992, [91]; Hammersley & Jourdain de Muizon 2001, [73]) and of the Cohen, Walker, Witeborn et al. (CWW) absolute calibration programme (Cohen et al. 1992a, [28]; 1992b, [29]; 1995, [31]; 1996, [32] and 1998, [33]), and more recently on detailed models provided by Decin et al. 2002, [41]; 2003a, [42]; 2003b, [43] and 2003c, [44]).
The long wavelength filters of PHT also use planets and asteroids to cover the bright and intermediate part of their dynamic range (Müller & Lagerros 1998, [125]; Griffin & Orton 1993, [64]; Orton 2001, [132]). The absolute flux calibration of LWS is based on Uranus as primary source (Sidher et al. 2001, [150]).
A catalogue of calibration source observations as well as many of the reference models used for the calibration of the four ISO instruments are available on-line at the ISO IDC web site:
http://www.iso.vilspa.esa.es/ 
ISO Explanatory Library
Performance and Calibration.