Let v(t, x) and u(t, x) be solutions of the heat equation v t −Δv = 0 and dissipative wave equation u tt +u t −Δu = 0, respectively. The paper finds the asymptotic expansions of the squared L 2 -norms of v, u and u − v as well as of their derivatives as t → ∞. Suitable conditions on the initial values u(0, x), u t (0, x) and v(0, x) lead to cancellation of the leading terms of the asymptotic expansion of u − v explaining the diffusion phenomenon for linear hyperbolic waves.