diff --git a/README.md b/README.md
index a842989..d753ebf 100644
--- a/README.md
+++ b/README.md
@@ -49,6 +49,15 @@ The waves were collectec over the following dates:
 * Wave 5: 2025-05-07 - 2025-05-20
 * Wave 6: 2025-07-02 - 2026-07-18
 
+## ChatGPT versions during data collection
+
+* May 2024: ChatGPT with GPT-3.5
+* Dec 2024: ChatGPT with GPT-4o and GPT-4o mini
+* May 2025: ChatGPT with GPT-4.1
+* Aug 2025: ChatGPT with GPT-5 and GPT-5 mini (after data collection)
+
+Source: https://en.wikipedia.org/wiki/ChatGPT#Model_versions
+
 ## Folder structure and files
 
 ```