To determine the influence of climate and litter quality on litter decomposition, the CIDET study followed litterbags of 10 foliar species at 18 upland forest sites and 3 wetland sites for 12 y. Changes in organic composition were characterized at 6 and 12 years for all litters from one site (Morgan Arboretum, MAR) with rapid initial decomposition (MAT 6.1° C), and for three litters at three colder sites (MAT, 4.2, 1.8 and -3.8° C). Decomposition slowed after the first few years, with some litters approaching an asymptote during 8-12 y. Species differences in percent carbon remaining (%C REM) also diminished with time. At MAR, %C REM was 16-38% at 6y and 12-25% at 12y, with higher %C REM at the colder sites. Proximate analysis at 0, 2, 4 and 6y (12y analyses are in progress) showed increases in the proportion of acid unhydrolysable residue (AUR). This is often incorrectly designated “lignin”, also incorporates but cutin and tannin structures. The AUR initially (up to 4 y) was lost more slowly or even increased slightly in some cases, but it was not more recalcitrant in the longer term. Solid-state ¹³C NMR at 6 y (both CP and quantitative BD) generally showed loss of O- and di-O-alkyl C, mainly from carbohydrate, and increase in other spectral regions. However, O-alkyl C loss was limited, especially for litters with slow initial decomposition. Many litters showed relatively small changes in intensity distribution, despite their high C losses, consistent with the small changes in DRIFT IR spectra and δ¹³C. Preliminary 12y NMR and DRIFT results show little further compositional change. Understanding what limits the later stages of decomposition will require interdisciplinary studies of how biota attack these complex, modified biopolymers, and hypotheses based on chemical science.