In kinetic models, gas-phase reactions involving large molecules (of more than about a dozen atoms) are often assumed to be in the high-pressure limit, that is, to not exhibit significant falloff or chemical activation effects. However, cases are known where rather large molecules are indeed involved in chemically activated, pressure-dependent reactions particularly at high temperatures. Here, we present simple formulas giving guidance for deciding whether a particular reaction's rate can be accurately approximated using the high-pressure limit rate constant k(infinity)(T). We find that under practical combustion conditions for liquid and gaseous fuels, few reactions are truly in the high-pressure limit, regardless of the size of the molecules involved.