In vitro methods were used to assess the full potential for decomposition (measured as biogas formation) from pit latrine samples taken from the top layer of 15 Tanzanian latrines. We found considerable variability in the decomposition rate and extent. This was compared with decomposition in the same latrines, measured by comparing top layer composition with fresh stools and deeper (older) layers, to assess whether this potential was realised in situ. Results showed a close match between the extent of organic material breakdown in situ and in vitro, indicating that anaerobic digestion is the dominant pathway in latrines. The average potential decrease in chemical oxygen demand (COD) (determined as methane production in vitro within 60 days) and actual measured decrease in situ are 68.9% ± 11.3 and 69.7% ± 19.4, respectively. However in the in vitro tests, where samples were diluted in water, full decomposition was achieved in 2 months, whereas in situ it can take years; this suggests that water addition may offer a simple route to improving latrine performance. The results also allowed us to estimate, for the first time to our knowledge using experimental data, the contribution that latrines make to greenhouse gas emissions globally. This amounts to ∼2% of annual US emissions.