We report the direct observation of one-dimensional (ID) excitonic molecules (biexcitons) in high quality disorder free semiconductor quantum wires. By means of spatially resolved near-field photoluminescence spectroscopy, we detected delocalized 1D excitons through the spatial extension of their homogeneous emission. 1D biexciton transitions are evidenced by their correlation to the 1D excitons as well as their power dependence. The photoluminescence intensity from biexcitons is shown to increase quadratically compared to the linear variation for excitonic emission, in agreement with the mass action law. The biexciton binding energy is measured to be 1.2 meV, and the lineshape is clearly asymmetric, showing the expected low energy tail. (C) 2002 Elsevier Science Ltd. All rights reserved.