Silicon is one of the most promising semiconductor materials for spin-based information processing devices(1,2). Its advanced fabrication technology facilitates the transition from individual devices to large-scale processors, and the availability of a Si-28 form with no magnetic nuclei overcomes a primary source of spin decoherence in many other materials(3,4). Nevertheless, the coherence lifetimes of electron spins in the solid state have typically remained several orders of magnitude lower than that achieved in isolated high-vacuum systems such as trapped ions(5). Here we examine electron spin coherence of donors in pure Si-28 material (residual Si-29 concentration <50 ppm) with donor densities of 10(14)-10(15) cm(-3). We elucidate three mechanisms for spin decoherence, active at different temperatures, and extract a coherence lifetime T-2 up to 2 s. In this regime, we find the electron spin is sensitive to interactions with other donor electron spins separated by similar to 200 nm. A magnetic field gradient suppresses such interactions, producing an extrapolated electron spin T-2 of 10 s at 1.8 K. These coherence lifetimes are without peer in the solid state and comparable to high-vacuum qubits, making electron spins of donors in silicon ideal components of quantum computers(2,6), or quantum memories for systems such as superconducting qubits(7-9).