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Optical multiplexing plays an important role in applications such as optical data storage, document security, molecular probes^, and bead assays for personalized medicine. Conventional fluorescent colour coding is limited by spectral overlap and background interference, restricting the number of distinguishable identities. Here, we show that tunable luminescent lifetimes τ in the microsecond region can be exploited to code individual upconversion nanocrystals. In a single colour band, one can generate more than ten nanocrystal populations with distinct lifetimes ranging from 25.6 µs to 662.4 µs and decode their well-separated lifetime identities, which are independent of both colour and intensity. Such ‘τ-dots’ potentially suit multichannel bioimaging, high-throughput cytometry quantification, high-density data storage, as well as security codes to combat counterfeiting. This demonstration extends the optical …