Formaldehyde (CH2O) is a characteristic species for the ignition phase of diesel-like fuels. As such, the spatio-temporal distribution of formaldehyde is an informative parameter in the study of the ignition event in internal combustion engines, especially for new combustion modes like homogeneous charge compression ignition (HCCI). This paper presents quantitative data on the CH2O distribution around diesel and n-heptane fuel sprays in the combustion chamber of a commercial heavy-duty diesel engine. Excitation of the 4(0)(1) band (355 nm) as well as the 4(0)(1)2(0)(1) band (339 nm) is applied. We use quantitative, spectrally resolved laser-induced fluorescence, calibrated by means of formalin seeding. to distinguish the contribution from CH2O to the signal from those of other species formed early in the combustion. Typically, between 40% and 100% of the fluorescence in the wavelength range considered characteristic for formaldehyde is in fact due to other species, but the latter are also related to the early combustion. Numerical simulation of a homogeneous reactor of n-heptane and air yields concentrations that are in reasonable agreement with the measurements. Formaldehyde starts to be formed at about 2 degrees CA (crank angle degrees) before the rise in main heat release. There appears to be a rather localised CH2O formation zone relatively close to the injector, out of which formaldehyde is transported downstream by the fuel jet. Once the hot combustion sets in, formaldehyde quickly disappears. (C) 2009 The Combustion Institute. Published by Elsevier Inc. All rights reserved.