New particle formation has been proposed to happen via activation of nanometer-sized condensation nuclei (nano-CN), which can be large molecules or molecular clusters. Electrically neutral nano-CN have mostly been outside the measurement range until recently. To address the concentrations and plausible chemical composition of the nano-CN, we measured the neutral particle size distribution down to mobility diameter 1.3 nm with a pulse-height CPC in Hyytiälä, southern Finland during springs 2007–2009. We also performed laboratory experiments with variable concentrations of precursors and oxidants in order to reproduce the atmospheric observations. The atmospheric nano-CN data were compared to ion size distributions (0.8–7.5 nm) measured with a BSMA ion spectrometer, and the mass spectra of natural ions detected with an APi-TOF mass spectrometer. We detected elevated concentrations of nano-CN, especially in the nocturnal boundary layer. However, the night-time nano-CN did not grow to sizes larger than a few nanometers. High temperature favored these night-time events, and the nano-CN concentration correlated well with the oxidation products of monoterpenes. The night-time negative ion mass spectra were dominated by large oxidized organic molecules and also the flow tube experiments support the idea that the nano-CN consist mostly of oxidized organic molecules. Nights with elevated neutral nano-CN concentration also showed enhanced concentrations of charged sulphuric acid monomer and dimer compared to other nights. However, all of the observed nano-CN does not necessarily participate to the atmospheric new particle formation process.