New particle formation (NPF) consists of two main steps: nucleation of gas- phase vapors and subsequent growth of the initial molecular clusters. While nucleation is mostly driven by inorganic acids, organic acids, such as oxidation products of α- pinene, are also known to contribute to the mass of fully grown particles. It is, however, not clear which organic molecules can nucleate in the absence of inorganic substances, or at what stage in NPF organic compounds become important. The reason is that current mass spectrometry techniques provide only compositional information and lack structural insights into the configurations of the initial-stage clusters. Here, we show that matrix-isolation Fourier transform infrared spectroscopy (MI-FTIR) can be used to study the initial steps of nucleation on a molecular level. Monomers and dimers of α-pinene oxidation products, such as pinic acid, are isolated in cryogenic argon matrices. By that, spectra are greatly simplified by the suppression of rotational sidebands. Our experiments are complemented by quantum chemical calculations, in which we calculate free energies and harmonic vibrational frequencies. By comparing their results to the experimental data, it is shown that, e.g., the measured dimer spectra of pinic acid fit best to those calculated for a dimer with a single OH-bridge. The existence of these dimer structures, which are not predicted to be the lowest free-energy conformer, could be ideal for subsequent growth. Up to three unbound OH groups are available for further oligomerization in singly-bridged dimers. In addition, our experiment shows that pinic acid forms dimers much more easily than, e.g., pinonic acid. By studying α-pinene oxidation products, we show that MI-FTIR is a versatile tool to investigate the structure of NPF precursor molecules and the very first steps of nucleation. The toolbox of MI-FTIR, in conjunction with theoretical calculations, can readily be applied to other organic NPF precursors.