Abstract. Even the simplest advection-diffusion problems can exhibit multiple time scales. This means that robust variable step time integrators are a prerequisite if such problems are to be efficiently solved computationally. The performance of the second order trapezoid rule using an explicit Adams-Bashforth method for error control is assessed in this work. This combination is particularly well suited to long time integration of advection-dominated problems. Herein it is shown that a stabilized implementation of the trapezoid rule leads to a very effective integrator in other situations: specifically diffusion problems with rough initial data; and general advection-diffusion problems with different physical time scales governing the system evolution.Key words. time-stepping, adaptivity, convection-diffusion AMS subject classifications. 65M12, 65M15, 65M20 DOI. 10.1137/0706880181. Background and context. The adaptive time-stepping algorithm that is the focus of this work is certainly not new. We consider the simplest Adams-BashforthMoulton pair. A version of our algorithm is hard-wired as the MATLAB function ode23t, see [24], and the underlying methodology is discussed in any many textbooks on the numerical solution of ODEs. See, for example, Henrici [13, p. 258] where estimation of the truncation error is discussed, or Iserles [16, p. 78], where stepdoubling and halving is described.The aim of this work is to assess the performance of this integrator in the context of method-of-lines discretization of PDEs that arise in incompressible flow modelling. In particular, we hope to provide insight into the role of adaptive time-stepping in the context of modelling multiple physical timescales. For this purpose it suffices to restrict our attention to the following simple model of scalar advection-diffusion:together with the initial condition u(x, 0) = u 0 (x), and boundary conditions (BCs)where a ≥ 0 (the advecting velocity), ν ≥ 0 (diffusivity), and u L and u R are given constants. In part II, we build on the foundation laid in this paper and consider the potential of the integrator in the context of solving the Navier-Stokes equations. *