This thesis describes the development of a Crossed Beam instrument for the study of ion-molecule reactions. A full investigation of some gross systematic errors, in apparatus which had been previously installed, is made. Where time has prevented the further development, then conclusions have been transformed into specifications, and practical solutions described. Experimental data is presented which is interpreted as evidence that the beams intersect and that low intensity of the scattered products and the forementioned systematic errors veil these data. The reduction of data, containing such errors, to useful chemical interpretation is extremely difficult, hence the need for further modification. Data are presented in chapter 3 consistent with the quadrupole acting as a linear particle accelerator. The effect on an initially monochromated beam is of causing an unacceptable energy spread. The spherical and chromatic aberrations of both source and detector are identified as the principal systematic errors using computer simulation (SIMION) for forward convolution. The development of a pulsed neutral beam source is described in chapter 4. The performance of the beam source is critically analysed and compared to alternative sources. A generally applicable model is derived to predict the shutter opening function of pulsed sources. Such a study has not previously been made. It is anticipated that this model will be of use to the solution of non-steady flow problems i.e. calculation of time dependent flow field properties, or in the deconvolution of TOF spectra from beam sources. Performance data for the beam source and some calculated properties are given. The subject of ion-molecule reactions is introduced by development from simple ideas. The subject matter covered is not intended to be exhaustive, but covers the examples of ion-molecule reactions of personal Interest. Recent crossed beam studies are examined and suggestions of possible new experiments made.