1-Pyrene carboxaldehyde (abbreviated as PyCHO), a non-amphiphilic derivative of pyrene has been successfully incorporated in Langmuir-Blodgett (LB) films, mixed with stearic acid (SA). Surface pressure (p) vs. area per molecule isotherms of PyCHO mixed with SA at different mole fractions reveal a plateau like region which indicates a phase transition corresponding to a reorientation of the PyCHO molecules at the air-water interface. Moreover, a positive deviation from the ideal behaviour of the area per molecule in different mole fraction in the mixed film at the air-water interface clearly reveals a repulsive interaction between the components resulting in the formation of aggregates of PyCHO molecules in the mixed films. Unlike pyrene molecules, formation of J-aggregates of PyCHO molecules in the mixed LB films has been confirmed by absorption spectroscopic studies. The fluorescence spectrum of the LB mixed film of PyCHO has been observed to lie between the solution and microcrystalline spectra and has a combination of both monomeric and excimeric species. A striking similarity between the LB film emission spectra and the emission spectra of PyCHO in ethanol-water binary mixture confirms the formation of low dimensional microcrystalline aggregates in the LB film.