| Acronym | Meaning |
| IPS | Inverse Photoemission Spectroscopy |
| IPSC | Inhibitory Postsynaptic Current |
| IPSP | Inhibitory Postsynaptic Potential |
| IPT | Iterated Perturbation Theory |
| IPVD | Ionized Physical Vapor Deposition |
| IPWO | Interface Plane Wave Orbitals |
| IQG | Independent Quasigeminals |
| IR | Isoprene Rubber |
| IRA | Interim Response Actions |
| IRAK | Interleukin 1 Receptor-associated Kinase |
| IRAS | Infrared Astronomy Satellite |
| IRAS | Infrared Reflection Absorption Spectroscopy |
| IRAV | Intense Infrared Active Vibration |
| IRBIC | Infrared Beam Induced Current |
| IRBP | Interstitial Retinol-binding Protein |
| IRC | Information Resource Center |
| IRC | Intrinsic Reaction Coordinate |
| IRCWD | International Reference Center For Waste Disposal |
| IRDO | Intermediate Retention Of Differential Overlap |
| IRE | Interferon-responsive Element |
| IRE | Internal Reflection Element |
| IREB | Intense Relativistic Electron Beam |
| IRED | IR Emitting Diode |
| IRED | Ion Radiation-enhanced Diffusion |
| IRES | Infrared Emission Spectroscopy |
| IRES | Internal Ribosome Entry Site |
| IRF | Instrument Response Function |
| IRF | Interferon Regulatory Factor |
| IRFPA | IR Focal Plane Array |
| IRGA | Infrared Gas Analyzer |
| IRGCHAZ | Intercritically Reheated Grain-coarsened Heat-affected Zone |
| IRGL | Interagency Regulatory Liaison Group |
| IRI | Industrial Research Institute |
| IRI | Interfaculty Reactor Institute |
| IRI | International Reference Ionosphere |
| IRI | Immunoreactive Insulin |
| IRIM | Institute For Rare Isotope Measurements |
| IRIS | Incorporated Research Institutes For Seismology |
| IRIS | Imaging Of Radicals Interacting With Surfaces |
| IRIS | Infrared Interferometer Spectrometer |
| IRIS | Integrated Risk Information System |
| IRK | Insulin Receptor Kinase |
| IRLG | Interagency Regulatory Liaison Group |
| IRM | Information Resources Management |
| IRM | Initial Remedial Measure |
| IRM | Isothermal Remanent Magnetization |
| IRMA | Immunoradiometric Assay |
| IRMDR | Infrared Microwave Double Resonance |
| IRMM | Institute For Reference Materials And Measurements |
| IRMP | International Remote Monitoring Program |