| Acronym | Meaning |
| IRMPA | Infrared Multiphoton Absorption |
| IRMPD | Infrared Multiphoton Dissociation |
| IRMS | Isotope Ratio Mass Spectrometry |
| IRN | Indicator Radionuclide |
| IRP | Installation Restoration Program |
| IRP | Internal Reflection Photolysis |
| IRPDS | Infrared Photothermal Deflection Spectroscopy |
| IRPTC | International Register Of Potentially Toxic Chemicals |
| IRR | Internal Rate Of Return |
| IRRAS | Infrared Reflection Absorption Spectroscopy |
| IRRAS | Integrated Reliability And Risk Analysis System |
| IRRI | International Rice Research Institute |
| IRRS | IR Reflection Spectroscopy |
| IRS | Infrared Soldering |
| IRS | Insulin Receptor Substrate |
| IRS | Interferon-responsive Sequence |
| IRS | Internal Recognition Subsequence |
| IRS | Internal Reflection Spectroscopy |
| IRS | Inverse Raman Scattering |
| IRSID | French Research Institute Of Siderurgy |
| IRSL | IR Stimulated Luminescence |
| IRTF | NASA Infrared Telescope Facility |
| IRZ | Immediate Response Zone |
| IS | Ionization Spectroscopy |
| ISA | Instrument Society Of America |
| ISA | Ionic Strength Adjuster |
| ISAAC | International Study Of Asthma And Allergies In Childhood |
| ISAM | Ionically Self-assembled Monolayer |
| ISAMS | Improved Stratospheric And Mesospheric Sounder |
| ISAS | Japanese Institute Of Space And Astronomical Science |
| ISAST | Irreversible Sorption Affected Solute Transport [model] |
| ISAT | In Situ Adaptive Tabulation |
| ISAW | Interactive Self-avoiding Walk [modeling] |
| ISBN | International Standard Book Number |
| ISC | Industrial Source Complex |
| ISC | Initial Safe Concentration |
| ISC | Intersystem Crossing |
| ISC | Short-circuit Current |
| ISC-PEM-MO | Iterative Self-consistent Partition Of Energy Method And MO Formalism |
| ISC3 | Industrial Source Complex Version 3 |
| ISCA | Ionization Spectroscopy For Chemical Analysis |
| ISCAT | Investigation Of Sulfur Chemistry In The Antarctic Troposphere |
| ISCC | Intergranular Stress-corrosion Cracking |
| ISCCP | International Satellite Cloud Climatology Project |
| ISCI | Initial-state Configuration Interaction |
| ISCLT | Industrial Source Complex Long Term (dispersion Model) |
| ISCN | International System For Human Cytogenetic Nomenclature |
| ISCRF | Inhomogeneous Self-consistent Reaction Field Theory |
| ISCST | Industrial Source Complex Short Term Model (US EPA) |
| ISD | Inclined Substrate Deposition |