C and N mineralization data for 17 different added organic materials (AOM) in a sandy soil were collected from an incubation experiment conducted under controlled laboratory conditions. The AOM originated from plants, animal wastes, manures, composts, and organic fertilizers. The C-to-N AOM ratios ([êta]AOM) ranged from 1.1 to 27.1. Sequential fibre analyses gave C-to-N ratios of soluble ([êta]Sol), holocellulosic ([êta]Hol) and ligneous compounds ([êta]Lig) ranging from 1.1 to 57.2, 0.8 to 65.2, and 3.5 to 25.3, respectively. Very different patterns of net AOM-N mineralization were observed: (i) immobilization for four plant AOM; (ii) moderate mineralization (4- 15% AOM-N) for composts; (iii) marked mineralization (11-27% AOM-N) for 1 animal AOM, 1 manure and 2 organic fertilizers; and (iv) high rates of transformations with possible gaseous losses for some N-rich AOM. The Transformation of Added Organics (TAO) model proposed here, described AOM-C mineralization (28°C, 75% WHC) from three labile (L), resistant (R) and stable (S) compartments with the sole parameters Pl and Ps = fractions of very labile and stable compounds of AOM, respectively. Dividing the C-compartments by their C-to-N estimates supplied the remaining NAOM fraction (RAONF). A Pim parameter split the TAO nitrogen fraction (TAONF = added N-RAONF) into two compartments, immobilized (imN) and inorganic (inorgN) N. A Pim > 0 value meant that all the TAONF plus a fraction (Pim - 1) of native soil inorganic N was immobilized. Additional N mineralization was predicted when necessary from imN by first order kinetics (constant kremim). The TAO version with two parameters Pim and kremin allowed us to predict very different patterns of N mineralization and N immobilization. In a few cases, a further first order kinetic law (constant kv) was added to predict N volatilization from inorgN. Two hypotheses were tested: (i) [êta]L, [êta]R, [êta]S (C-to-N of L, R and S) [êta]Sol, [êta]Hol, [êta]Lig, respectively